diff --git a/packages/babel-plugin-transform-es2015-constants/.npmignore b/packages/babel-plugin-check-es2015-constants/.npmignore
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/.npmignore
rename to packages/babel-plugin-check-es2015-constants/.npmignore
diff --git a/packages/babel-plugin-check-es2015-constants/README.md b/packages/babel-plugin-check-es2015-constants/README.md
new file mode 100644
index 0000000000..0bd2c3b4ab
--- /dev/null
+++ b/packages/babel-plugin-check-es2015-constants/README.md
@@ -0,0 +1,39 @@
+# babel-plugin-check-es2015-constants
+
+Validate ES2015 constants
+
+## Installation
+
+```sh
+$ npm install babel-plugin-check-es2015-constants
+```
+
+## Usage
+
+### Via `.babelrc` (Recommended)
+
+**.babelrc**
+
+```json
+{
+  "plugins": ["check-es2015-constants"]
+}
+```
+
+### Via CLI
+
+```sh
+$ babel --plugins check-es2015-constants script.js
+```
+
+### Via Node API
+
+```javascript
+require("babel-core").transform("code", {
+  plugins: ["check-es2015-constants"]
+});
+```
+
+## Note
+
+This check will only validate consts. If you need it to compile down to `var` then you must also install and enable [`check-es2015-block-scoping`](../babel-plugin-check-es2015-block-scoping).
diff --git a/packages/babel-plugin-transform-es2015-constants/package.json b/packages/babel-plugin-check-es2015-constants/package.json
similarity index 78%
rename from packages/babel-plugin-transform-es2015-constants/package.json
rename to packages/babel-plugin-check-es2015-constants/package.json
index 85fb9e267b..4942be62d0 100644
--- a/packages/babel-plugin-transform-es2015-constants/package.json
+++ b/packages/babel-plugin-check-es2015-constants/package.json
@@ -1,8 +1,8 @@
 {
-  "name": "babel-plugin-transform-es2015-constants",
+  "name": "babel-plugin-check-es2015-constants",
   "version": "6.1.4",
   "description": "Compile ES2015 constants to ES5",
-  "repository": "https://github.com/babel/babel/tree/master/packages/babel-plugin-transform-es2015-constants",
+  "repository": "https://github.com/babel/babel/tree/master/packages/babel-plugin-check-es2015-constants",
   "license": "MIT",
   "main": "lib/index.js",
   "keywords": [
diff --git a/packages/babel-plugin-check-es2015-constants/src/index.js b/packages/babel-plugin-check-es2015-constants/src/index.js
new file mode 100644
index 0000000000..4269b0385c
--- /dev/null
+++ b/packages/babel-plugin-check-es2015-constants/src/index.js
@@ -0,0 +1,16 @@
+export default function ({ messages }) {
+  return {
+    visitor: {
+      Scope({ scope }) {
+        for (let name in scope.bindings) {
+          let binding = scope.bindings[name];
+          if (binding.kind !== "const" && binding.kind !== "module") continue;
+
+          for (let violation of (binding.constantViolations: Array)) {
+            throw violation.buildCodeFrameError(messages.get("readOnly", name));
+          }
+        }
+      },
+    }
+  };
+}
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/block-scoped/exec.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/block-scoped/exec.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/block-scoped/exec.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/block-scoped/exec.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/block-statement/actual.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/block-statement/actual.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/block-statement/actual.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/block-statement/actual.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/block-statement/expected.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/block-statement/expected.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/block-statement/expected.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/block-statement/expected.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/destructuring-assignment/actual.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/destructuring-assignment/actual.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/destructuring-assignment/actual.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/destructuring-assignment/actual.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/destructuring-assignment/options.json b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/destructuring-assignment/options.json
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/destructuring-assignment/options.json
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/destructuring-assignment/options.json
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/destructuring/actual.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/destructuring/actual.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/destructuring/actual.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/destructuring/actual.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/destructuring/expected.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/destructuring/expected.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/destructuring/expected.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/destructuring/expected.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/ignore-member-expressions/actual.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/ignore-member-expressions/actual.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/ignore-member-expressions/actual.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/ignore-member-expressions/actual.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/ignore-member-expressions/expected.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/ignore-member-expressions/expected.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/ignore-member-expressions/expected.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/ignore-member-expressions/expected.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/loop/actual.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/loop/actual.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/loop/actual.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/loop/actual.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/loop/options.json b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/loop/options.json
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/loop/options.json
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/loop/options.json
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-assignment/actual.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-assignment/actual.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-assignment/actual.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-assignment/actual.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-assignment/options.json b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-assignment/options.json
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-assignment/options.json
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-assignment/options.json
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-classes/actual.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-classes/actual.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-classes/actual.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-classes/actual.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-classes/options.json b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-classes/options.json
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-classes/options.json
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-classes/options.json
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-declaration/actual.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-declaration/actual.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-declaration/actual.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-declaration/actual.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-declaration/options.json b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-declaration/options.json
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-declaration/options.json
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-declaration/options.json
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-for-in/actual.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-for-in/actual.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-for-in/actual.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-for-in/actual.js
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similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-for-in/options.json
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-for-in/options.json
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-functions/actual.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-functions/actual.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-functions/actual.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-functions/actual.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-functions/options.json b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-functions/options.json
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/no-functions/options.json
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/no-functions/options.json
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/program/actual.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/program/actual.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/program/actual.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/program/actual.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/program/expected.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/program/expected.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/program/expected.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/program/expected.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/update-expression/actual.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/update-expression/actual.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/update-expression/actual.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/update-expression/actual.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/update-expression/expected.js b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/update-expression/expected.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/update-expression/expected.js
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/update-expression/expected.js
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/general/update-expression/options.json b/packages/babel-plugin-check-es2015-constants/test/fixtures/general/update-expression/options.json
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/fixtures/general/update-expression/options.json
rename to packages/babel-plugin-check-es2015-constants/test/fixtures/general/update-expression/options.json
diff --git a/packages/babel-plugin-check-es2015-constants/test/fixtures/options.json b/packages/babel-plugin-check-es2015-constants/test/fixtures/options.json
new file mode 100644
index 0000000000..c0e516303a
--- /dev/null
+++ b/packages/babel-plugin-check-es2015-constants/test/fixtures/options.json
@@ -0,0 +1,3 @@
+{
+  "plugins": ["check-es2015-constants", "transform-es2015-block-scoping", "transform-es2015-destructuring"]
+}
diff --git a/packages/babel-plugin-transform-es2015-constants/test/index.js b/packages/babel-plugin-check-es2015-constants/test/index.js
similarity index 100%
rename from packages/babel-plugin-transform-es2015-constants/test/index.js
rename to packages/babel-plugin-check-es2015-constants/test/index.js
diff --git a/packages/babel-plugin-transform-es2015-block-scoping/src/index.js b/packages/babel-plugin-transform-es2015-block-scoping/src/index.js
index 4354217f98..f0347595d6 100644
--- a/packages/babel-plugin-transform-es2015-block-scoping/src/index.js
+++ b/packages/babel-plugin-transform-es2015-block-scoping/src/index.js
@@ -13,9 +13,10 @@ export default function () {
     visitor: {
       VariableDeclaration(path, file) {
         let { node, parent, scope } = path;
-        if (!isLet(node, parent, scope)) return;
+        if (!isBlockScoped(node)) return;
+        convertBlockScopedToVar(node, parent, scope);
 
-        if (isLetInitable(node) && node._tdzThis) {
+        if (node._tdzThis) {
           let nodes = [node];
 
           for (let i = 0; i < node.declarations.length; i++) {
@@ -42,13 +43,7 @@ export default function () {
 
       Loop(path, file) {
         let { node, parent, scope } = path;
-
-        let init = node.left || node.init;
-        if (isLet(init, node, scope)) {
-          t.ensureBlock(node);
-          node.body._letDeclarators = [init];
-        }
-
+        t.ensureBlock(node);
         let blockScoping = new BlockScoping(path, path.get("body"), parent, scope, file);
         let replace = blockScoping.run();
         if (replace) path.replaceWith(replace);
@@ -68,36 +63,28 @@ let buildRetCheck = template(`
   if (typeof RETURN === "object") return RETURN.v;
 `);
 
-function isLet(node, parent, scope) {
+function isBlockScoped(node) {
   if (!t.isVariableDeclaration(node)) return false;
-  if (node._let) return true;
-  if (node.kind !== "let") return false;
+  if (node[t.BLOCK_SCOPED_SYMBOL]) return true;
+  if (node.kind !== "let" && node.kind !== "const") return false;
+  return true;
+}
 
+function convertBlockScopedToVar(node, parent, scope) {
   // https://github.com/babel/babel/issues/255
-  if (isLetInitable(node, parent)) {
+  if (!t.isFor(parent)) {
     for (let i = 0; i < node.declarations.length; i++) {
       let declar = node.declarations[i];
       declar.init = declar.init || scope.buildUndefinedNode();
     }
   }
 
-  node._let = true;
+  node[t.BLOCK_SCOPED_SYMBOL] = true;
   node.kind = "var";
-  return true;
 }
 
-function isLetInitable(node, parent) {
-  return !t.isFor(parent) || !t.isFor(parent, { left: node });
-}
-
-function isVar(node, parent, scope) {
-  return t.isVariableDeclaration(node, { kind: "var" }) && !isLet(node, parent, scope);
-}
-
-function standardizeLets(declars) {
-  for (let declar of (declars: Array)) {
-    delete declar._let;
-  }
+function isVar(node) {
+  return t.isVariableDeclaration(node, { kind: "var" }) && !isBlockScoped(node);
 }
 
 function replace(path, node, scope, remaps) {
@@ -167,10 +154,10 @@ let letReferenceFunctionVisitor = traverse.visitors.merge([{
 
 let hoistVarDeclarationsVisitor = {
   enter(path, self) {
-    let { node, parent, scope } = path;
+    let { node, parent } = path;
 
     if (path.isForStatement()) {
-      if (isVar(node.init, node, scope)) {
+      if (isVar(node.init, node)) {
         let nodes = self.pushDeclar(node.init);
         if (nodes.length === 1) {
           node.init = nodes[0];
@@ -179,11 +166,11 @@ let hoistVarDeclarationsVisitor = {
         }
       }
     } else if (path.isFor()) {
-      if (isVar(node.left, node, scope)) {
+      if (isVar(node.left, node)) {
         self.pushDeclar(node.left);
         node.left = node.left.declarations[0].id;
       }
-    } else if (isVar(node, parent, scope)) {
+    } else if (isVar(node, parent)) {
       path.replaceWithMultiple(self.pushDeclar(node).map(expr => t.expressionStatement(expr)));
     } else if (path.isFunction()) {
       return path.skip();
@@ -498,19 +485,22 @@ class BlockScoping {
   getLetReferences() {
     let block = this.block;
 
-    let declarators = block._letDeclarators || [];
+    let declarators = [];
 
-    //
-    for (let i = 0; i < declarators.length; i++) {
-      let declar = declarators[i];
-      extend(this.outsideLetReferences, t.getBindingIdentifiers(declar));
+    if (this.loop) {
+      let init = this.loop.left || this.loop.init;
+      if (isBlockScoped(init)) {
+        declarators.push(init);
+        extend(this.outsideLetReferences, t.getBindingIdentifiers(init));
+      }
     }
 
     //
     if (block.body) {
       for (let i = 0; i < block.body.length; i++) {
         let declar = block.body[i];
-        if (t.isClassDeclaration(declar) || t.isFunctionDeclaration(declar) || isLet(declar, block, this.scope)) {
+        if (t.isClassDeclaration(declar) || t.isFunctionDeclaration(declar) || isBlockScoped(declar)) {
+          if (isBlockScoped(declar)) convertBlockScopedToVar(declar, block, this.scope);
           declarators = declarators.concat(declar.declarations || declar);
         }
       }
@@ -527,9 +517,6 @@ class BlockScoping {
     // no let references so we can just quit
     if (!this.hasLetReferences) return;
 
-    // set let references to plain let references
-    standardizeLets(declarators);
-
     let state = {
       letReferences: this.letReferences,
       closurify:     false,
diff --git a/packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/general/assignment-patterns/expected.js b/packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/general/assignment-patterns/expected.js
index b497a041e4..41c81d3bb4 100644
--- a/packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/general/assignment-patterns/expected.js
+++ b/packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/general/assignment-patterns/expected.js
@@ -1,4 +1,4 @@
-const foo = "foo";
+var foo = "foo";
 
 function foobar() {
   for (var item of [1, 2, 3]) {
diff --git a/packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/general/for-const-closure/actual.js b/packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/general/for-const-closure/actual.js
new file mode 100644
index 0000000000..55dbb21d37
--- /dev/null
+++ b/packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/general/for-const-closure/actual.js
@@ -0,0 +1,6 @@
+for (let i = 0; i < 5; i++) {
+  const l = i;
+  setTimeout(function() {
+    console.log(l);
+  }, 1);
+}
diff --git a/packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/general/for-const-closure/expected.js b/packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/general/for-const-closure/expected.js
new file mode 100644
index 0000000000..5b83611a8b
--- /dev/null
+++ b/packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/general/for-const-closure/expected.js
@@ -0,0 +1,10 @@
+var _loop = function (i) {
+  var l = i;
+  setTimeout(function () {
+    console.log(l);
+  }, 1);
+};
+
+for (var i = 0; i < 5; i++) {
+  _loop(i);
+}
diff --git a/packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/pass/options.json b/packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/pass/options.json
index 7c2bea4aad..a0ae43ffad 100644
--- a/packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/pass/options.json
+++ b/packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/pass/options.json
@@ -1,3 +1,3 @@
 {
-  "plugins": ["transform-es2015-constants", "transform-es2015-block-scoping", "transform-es2015-parameters", "transform-es2015-destructuring", "transform-es2015-modules-commonjs"]
+  "plugins": ["check-es2015-constants", "transform-es2015-block-scoping", "transform-es2015-parameters", "transform-es2015-destructuring", "transform-es2015-modules-commonjs"]
 }
diff --git a/packages/babel-plugin-transform-es2015-constants/README.md b/packages/babel-plugin-transform-es2015-constants/README.md
deleted file mode 100644
index 33c2610514..0000000000
--- a/packages/babel-plugin-transform-es2015-constants/README.md
+++ /dev/null
@@ -1,39 +0,0 @@
-# babel-plugin-transform-es2015-constants
-
-Compile ES2015 constants to ES5
-
-## Installation
-
-```sh
-$ npm install babel-plugin-transform-es2015-constants
-```
-
-## Usage
-
-### Via `.babelrc` (Recommended)
-
-**.babelrc**
-
-```json
-{
-  "plugins": ["transform-es2015-constants"]
-}
-```
-
-### Via CLI
-
-```sh
-$ babel --plugins transform-es2015-constants script.js
-```
-
-### Via Node API
-
-```javascript
-require("babel-core").transform("code", {
-  plugins: ["transform-es2015-constants"]
-});
-```
-
-## Note
-
-This transform on its own will compile `const` to `let`. If you need it to compile down to `var` then you must also install and enable [`transform-es2015-block-scoping`](../babel-plugin-transform-es2015-block-scoping).
diff --git a/packages/babel-plugin-transform-es2015-constants/src/index.js b/packages/babel-plugin-transform-es2015-constants/src/index.js
deleted file mode 100644
index 7970a0ad89..0000000000
--- a/packages/babel-plugin-transform-es2015-constants/src/index.js
+++ /dev/null
@@ -1,38 +0,0 @@
-export default function ({ messages, types: t }) {
-  function check(node) {
-    if (t.isVariableDeclaration(node, { kind: "const" })) {
-      node.kind = "let";
-    }
-  }
-
-  return {
-    visitor: {
-      Scope({ scope }) {
-        for (let name in scope.bindings) {
-          let binding = scope.bindings[name];
-          if (binding.kind !== "const" && binding.kind !== "module") continue;
-
-          for (let violation of (binding.constantViolations: Array)) {
-            throw violation.buildCodeFrameError(messages.get("readOnly", name));
-          }
-        }
-      },
-
-      VariableDeclaration({ node }) {
-        check(node);
-      },
-
-      ForXStatement({ node: { left } }) {
-        check(left);
-      },
-
-      ForStatement({ node: { init } }) {
-        check(init);
-      },
-
-      "BlockStatement|Program"({ node: { body } }) {
-        for (let node of body) check(node);
-      }
-    }
-  };
-}
diff --git a/packages/babel-plugin-transform-es2015-constants/test/fixtures/options.json b/packages/babel-plugin-transform-es2015-constants/test/fixtures/options.json
deleted file mode 100644
index b4063f1b64..0000000000
--- a/packages/babel-plugin-transform-es2015-constants/test/fixtures/options.json
+++ /dev/null
@@ -1,3 +0,0 @@
-{
-  "plugins": ["transform-es2015-constants", "transform-es2015-block-scoping", "transform-es2015-destructuring"]
-}
diff --git a/packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/rest-binding-deoptimisation/expected.js b/packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/rest-binding-deoptimisation/expected.js
index 80224dbe80..af7c898bdc 100644
--- a/packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/rest-binding-deoptimisation/expected.js
+++ b/packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/rest-binding-deoptimisation/expected.js
@@ -1,4 +1,4 @@
-const deepAssign = function () {
+var deepAssign = function () {
   for (var _len = arguments.length, args = Array(_len), _key = 0; _key < _len; _key++) {
     args[_key] = arguments[_key];
   }
diff --git a/packages/babel-preset-es2015/index.js b/packages/babel-preset-es2015/index.js
index d30d1461ad..f8fdb617e4 100644
--- a/packages/babel-preset-es2015/index.js
+++ b/packages/babel-preset-es2015/index.js
@@ -12,7 +12,7 @@ module.exports = {
     require("babel-plugin-transform-es2015-for-of"),
     require("babel-plugin-transform-es2015-sticky-regex"),
     require("babel-plugin-transform-es2015-unicode-regex"),
-    require("babel-plugin-transform-es2015-constants"),
+    require("babel-plugin-check-es2015-constants"),
     require("babel-plugin-transform-es2015-spread"),
     require("babel-plugin-transform-es2015-parameters"),
     require("babel-plugin-transform-es2015-destructuring"),
diff --git a/packages/babel-preset-es2015/package.json b/packages/babel-preset-es2015/package.json
index d1aac6653f..b8e59870b7 100644
--- a/packages/babel-preset-es2015/package.json
+++ b/packages/babel-preset-es2015/package.json
@@ -20,7 +20,7 @@
     "babel-plugin-transform-es2015-for-of": "^6.1.4",
     "babel-plugin-transform-es2015-sticky-regex": "^6.1.4",
     "babel-plugin-transform-es2015-unicode-regex": "^6.1.4",
-    "babel-plugin-transform-es2015-constants": "^6.1.4",
+    "babel-plugin-check-es2015-constants": "^6.1.4",
     "babel-plugin-transform-es2015-spread": "^6.1.4",
     "babel-plugin-transform-es2015-parameters": "^6.1.4",
     "babel-plugin-transform-es2015-destructuring": "^6.1.4",
diff --git a/packages/babel-preset-es2015/test/fixtures/traceur/options.json b/packages/babel-preset-es2015/test/fixtures/traceur/options.json
index 668b348e59..f549ded098 100644
--- a/packages/babel-preset-es2015/test/fixtures/traceur/options.json
+++ b/packages/babel-preset-es2015/test/fixtures/traceur/options.json
@@ -14,7 +14,7 @@
     "transform-es2015-for-of",
     "transform-es2015-sticky-regex",
     "transform-es2015-unicode-regex",
-    "transform-es2015-constants",
+    "check-es2015-constants",
     "transform-es2015-spread",
     "transform-es2015-parameters",
     "transform-es2015-destructuring",
diff --git a/packages/babel-types/src/constants.js b/packages/babel-types/src/constants.js
index 7cccc83192..275b6cfcf5 100644
--- a/packages/babel-types/src/constants.js
+++ b/packages/babel-types/src/constants.js
@@ -22,3 +22,5 @@ export const INHERIT_KEYS = {
   optional: ["typeAnnotation", "typeParameters", "returnType"],
   force: ["start", "loc", "end"]
 };
+
+export const BLOCK_SCOPED_SYMBOL = Symbol.for("var used to be block scoped");
diff --git a/packages/babel-types/src/validators.js b/packages/babel-types/src/validators.js
index 6460b9ae1f..019cec3f0b 100644
--- a/packages/babel-types/src/validators.js
+++ b/packages/babel-types/src/validators.js
@@ -3,6 +3,7 @@
 import { getBindingIdentifiers } from "./retrievers";
 import esutils from "esutils";
 import * as t from "./index";
+import { BLOCK_SCOPED_SYMBOL } from "./constants";
 
 /**
  * Check if the input `node` is a binding identifier.
@@ -169,7 +170,7 @@ export function isValidIdentifier(name: string): boolean {
  */
 
 export function isLet(node: Object): boolean {
-  return t.isVariableDeclaration(node) && (node.kind !== "var" || node._let);
+  return t.isVariableDeclaration(node) && (node.kind !== "var" || node[BLOCK_SCOPED_SYMBOL]);
 }
 
 /**
@@ -185,7 +186,7 @@ export function isBlockScoped(node: Object): boolean {
  */
 
 export function isVar(node: Object): boolean {
-  return t.isVariableDeclaration(node, { kind: "var" }) && !node._let;
+  return t.isVariableDeclaration(node, { kind: "var" }) && !node[BLOCK_SCOPED_SYMBOL];
 }
 
 /**