diff --git a/lib/6to5/file.js b/lib/6to5/file.js
index 406a919b1f..ba4ed2faaf 100644
--- a/lib/6to5/file.js
+++ b/lib/6to5/file.js
@@ -9,6 +9,7 @@ var transform  = require("./transformation");
 var generate   = require("./generation");
 var defaults   = require("lodash/object/defaults");
 var contains   = require("lodash/collection/contains");
+var traverse   = require("./traverse");
 var clone      = require("./helpers/clone");
 var Scope      = require("./traverse/scope");
 var util       = require("./util");
@@ -352,6 +353,8 @@ File.prototype.transform = function (ast) {
     modFormatter.init();
   }
 
+  this.checkNode(ast);
+
   var astRun = function (key) {
     each(self.transformerStack, function (pass) {
       pass.astRun(key);
@@ -367,6 +370,30 @@ File.prototype.transform = function (ast) {
   astRun("exit");
 };
 
+var checkTransformerVisitor = {
+  enter: function (node, parent, scope, context, state) {
+    state.check(node, scope);
+  }
+};
+
+File.prototype.checkNode = function (node, scope) {
+  var self = this;
+  scope = scope || this.scope;
+
+  var check = function (node, scope) {
+    each(self.transformerStack, function (pass) {
+      if (pass.shouldRun) return;
+      pass.checkNode(node, scope);
+    });
+  };
+
+  check(node, scope);
+
+  traverse(node, checkTransformerVisitor, scope, {
+    check: check
+  });
+};
+
 File.prototype.generate = function () {
   var opts = this.opts;
   var ast  = this.ast;
diff --git a/lib/6to5/generation/generators/types.js b/lib/6to5/generation/generators/types.js
index 117af92ea1..999cd4ae2d 100644
--- a/lib/6to5/generation/generators/types.js
+++ b/lib/6to5/generation/generators/types.js
@@ -94,8 +94,10 @@ exports.Literal = function (node) {
     });
 
     this.push(val);
-  } else if (type === "boolean" || type === "number") {
-    this.push(JSON.stringify(val));
+  } else if (type === "number") {
+    this.push(val + "");
+  } else if (type === "boolean" ) {
+    this.push(val ? "true" : "false");
   } else if (node.regex) {
     this.push("/" + node.regex.pattern + "/" + node.regex.flags);
   } else if (val === null) {
diff --git a/lib/6to5/transformation/transformer-pass.js b/lib/6to5/transformation/transformer-pass.js
index 95a013f68f..5c02d6e215 100644
--- a/lib/6to5/transformation/transformer-pass.js
+++ b/lib/6to5/transformation/transformer-pass.js
@@ -11,11 +11,14 @@ var contains = require("lodash/collection/contains");
 
 function TransformerPass(file, transformer) {
   this.transformer = transformer;
+  this.shouldRun   = !transformer.check;
   this.handlers    = transformer.handlers;
   this.file        = file;
 }
 
 TransformerPass.prototype.astRun = function (key) {
+  if (!this.shouldRun) return;
+
   var handlers = this.handlers;
   var file = this.file;
 
@@ -29,23 +32,39 @@ TransformerPass.prototype.canRun = function () {
 
   var opts = this.file.opts;
   var key  = transformer.key;
+
+  // internal
   if (key[0] === "_") return true;
 
+  // blacklist
   var blacklist = opts.blacklist;
   if (blacklist.length && contains(blacklist, key)) return false;
 
+  // whitelist
   var whitelist = opts.whitelist;
   if (whitelist.length && !contains(whitelist, key)) return false;
 
+  // optional
   if (transformer.optional && !contains(opts.optional, key)) return false;
 
+  // experimental
   if (transformer.experimental && !opts.experimental) return false;
 
+  // playground
   if (transformer.playground && !opts.playground) return false;
 
   return true;
 };
 
+TransformerPass.prototype.checkNode = function (node) {
+  var check = this.transformer.check;
+  if (check) {
+    return this.shouldRun = check(node);
+  } else {
+    return true;
+  }
+};
+
 var transformVisitor = {
   enter: function (node, parent, scope, context, state) {
     var fns = state.handlers[node.type];
@@ -61,6 +80,8 @@ var transformVisitor = {
 };
 
 TransformerPass.prototype.transform = function () {
+  if (!this.shouldRun) return;
+
   var file = this.file;
 
   util.debug(file.opts.filename + ": Running transformer " + this.transformer.key);
diff --git a/lib/6to5/transformation/transformer.js b/lib/6to5/transformation/transformer.js
index 02efb7396e..ce4a87e3d9 100644
--- a/lib/6to5/transformation/transformer.js
+++ b/lib/6to5/transformation/transformer.js
@@ -16,13 +16,16 @@ var each            = require("lodash/collection/each");
 
 function Transformer(key, transformer, opts) {
   this.manipulateOptions = transformer.manipulateOptions;
-  this.experimental      = !!transformer.experimental;
-  this.playground        = !!transformer.playground;
-  this.secondPass        = !!transformer.secondPass;
-  this.optional          = !!transformer.optional;
-  this.handlers          = this.normalize(transformer);
-  this.opts              = opts || {};
-  this.key               = key;
+  this.check             = transformer.check;
+
+  this.experimental = !!transformer.experimental;
+  this.playground   = !!transformer.playground;
+  this.secondPass   = !!transformer.secondPass;
+  this.optional     = !!transformer.optional;
+
+  this.handlers = this.normalize(transformer);
+  this.opts     = opts || {};
+  this.key      = key;
 }
 
 Transformer.prototype.normalize = function (transformer) {
diff --git a/lib/6to5/transformation/transformers/es5/properties.mutators.js b/lib/6to5/transformation/transformers/es5/properties.mutators.js
index 40b87e6fe4..b939b160da 100644
--- a/lib/6to5/transformation/transformers/es5/properties.mutators.js
+++ b/lib/6to5/transformation/transformers/es5/properties.mutators.js
@@ -3,6 +3,10 @@
 var defineMap = require("../../helpers/define-map");
 var t         = require("../../../types");
 
+exports.check = function (node) {
+  return t.isProperty(node) && (node.kind === "get" || node.kind === "set");
+};
+
 exports.ObjectExpression = function (node) {
   var mutatorMap = {};
   var hasAny = false;
diff --git a/lib/6to5/transformation/transformers/es6/arrow-functions.js b/lib/6to5/transformation/transformers/es6/arrow-functions.js
index 32dec4e25c..aa62f9ef21 100644
--- a/lib/6to5/transformation/transformers/es6/arrow-functions.js
+++ b/lib/6to5/transformation/transformers/es6/arrow-functions.js
@@ -2,6 +2,8 @@
 
 var t = require("../../../types");
 
+exports.check = t.isArrowFunctionExpression;
+
 exports.ArrowFunctionExpression = function (node) {
   t.ensureBlock(node);
 
diff --git a/lib/6to5/transformation/transformers/es6/block-scoping.js b/lib/6to5/transformation/transformers/es6/block-scoping.js
index def499aa33..40daed0dae 100644
--- a/lib/6to5/transformation/transformers/es6/block-scoping.js
+++ b/lib/6to5/transformation/transformers/es6/block-scoping.js
@@ -7,6 +7,10 @@ var t        = require("../../../types");
 var values   = require("lodash/object/values");
 var extend   = require("lodash/object/extend");
 
+exports.check = function (node) {
+  return t.isVariableDeclaration(node) && (node.kind === "let" || node.kind === "const");
+};
+
 var isLet = function (node, parent) {
   if (!t.isVariableDeclaration(node)) return false;
   if (node._let) return true;
diff --git a/lib/6to5/transformation/transformers/es6/classes.js b/lib/6to5/transformation/transformers/es6/classes.js
index 2ede1861a8..61e906ca62 100644
--- a/lib/6to5/transformation/transformers/es6/classes.js
+++ b/lib/6to5/transformation/transformers/es6/classes.js
@@ -6,6 +6,8 @@ var defineMap     = require("../../helpers/define-map");
 var util          = require("../../../util");
 var t             = require("../../../types");
 
+exports.check = t.isClass;
+
 exports.ClassDeclaration = function (node, parent, scope, context, file) {
   return new Class(node, file, scope, true).run();
 };
diff --git a/lib/6to5/transformation/transformers/es6/constants.js b/lib/6to5/transformation/transformers/es6/constants.js
index dcedaa222f..cb7101faf2 100644
--- a/lib/6to5/transformation/transformers/es6/constants.js
+++ b/lib/6to5/transformation/transformers/es6/constants.js
@@ -3,6 +3,10 @@
 var traverse = require("../../../traverse");
 var t        = require("../../../types");
 
+exports.check = function (node) {
+  return t.isVariableDeclaration(node, { kind: "const" });
+};
+
 var visitor = {
   enter: function (node, parent, scope, context, state) {
     if (t.isAssignmentExpression(node) || t.isUpdateExpression(node)) {
diff --git a/lib/6to5/transformation/transformers/es6/destructuring.js b/lib/6to5/transformation/transformers/es6/destructuring.js
index 246edf31ee..6dc61bfd6e 100644
--- a/lib/6to5/transformation/transformers/es6/destructuring.js
+++ b/lib/6to5/transformation/transformers/es6/destructuring.js
@@ -4,6 +4,8 @@
 
 var t = require("../../../types");
 
+exports.check = t.isPattern;
+
 var buildVariableAssign = function (opts, id, init) {
   var op = opts.operator;
   if (t.isMemberExpression(id)) op = "=";
diff --git a/lib/6to5/transformation/transformers/es6/for-of.js b/lib/6to5/transformation/transformers/es6/for-of.js
index 0bbfd9fc83..938ab0df42 100644
--- a/lib/6to5/transformation/transformers/es6/for-of.js
+++ b/lib/6to5/transformation/transformers/es6/for-of.js
@@ -3,6 +3,8 @@
 var util = require("../../../util");
 var t    = require("../../../types");
 
+exports.check = t.isForOfStatement;
+
 exports.ForOfStatement = function (node, parent, scope, context, file) {
   var callback = spec;
   if (file.isLoose("es6.forOf")) callback = loose;
diff --git a/lib/6to5/transformation/transformers/es6/modules.js b/lib/6to5/transformation/transformers/es6/modules.js
index abb911f0b5..9249b3da64 100644
--- a/lib/6to5/transformation/transformers/es6/modules.js
+++ b/lib/6to5/transformation/transformers/es6/modules.js
@@ -2,6 +2,8 @@
 
 var t = require("../../../types");
 
+exports.check = require("../internal/modules").check;
+
 exports.ImportDeclaration = function (node, parent, scope, context, file) {
   var nodes = [];
 
diff --git a/lib/6to5/transformation/transformers/es6/parameters.default.js b/lib/6to5/transformation/transformers/es6/parameters.default.js
index 20184cfee9..2854e572d7 100644
--- a/lib/6to5/transformation/transformers/es6/parameters.default.js
+++ b/lib/6to5/transformation/transformers/es6/parameters.default.js
@@ -4,6 +4,10 @@ var traverse = require("../../../traverse");
 var util     = require("../../../util");
 var t        = require("../../../types");
 
+exports.check = function (node) {
+  return t.isFunction(node) && hasDefaults(node);
+};
+
 var hasDefaults = function (node) {
   for (var i = 0; i < node.params.length; i++) {
     if (t.isAssignmentPattern(node.params[i])) return true;
diff --git a/lib/6to5/transformation/transformers/es6/parameters.rest.js b/lib/6to5/transformation/transformers/es6/parameters.rest.js
index b885ed6e1d..60a8aca8b2 100644
--- a/lib/6to5/transformation/transformers/es6/parameters.rest.js
+++ b/lib/6to5/transformation/transformers/es6/parameters.rest.js
@@ -3,6 +3,8 @@
 var util = require("../../../util");
 var t    = require("../../../types");
 
+exports.check = t.isRestElement;
+
 var hasRest = function (node) {
   return t.isRestElement(node.params[node.params.length - 1]);
 };
diff --git a/lib/6to5/transformation/transformers/es6/properties.computed.js b/lib/6to5/transformation/transformers/es6/properties.computed.js
index 5091488ca7..1b557a6d35 100644
--- a/lib/6to5/transformation/transformers/es6/properties.computed.js
+++ b/lib/6to5/transformation/transformers/es6/properties.computed.js
@@ -2,6 +2,10 @@
 
 var t = require("../../../types");
 
+exports.check = function (node) {
+  return t.isProperty(node) && node.computed;
+};
+
 exports.ObjectExpression = function (node, parent, scope, context, file) {
   var hasComputed = false;
 
diff --git a/lib/6to5/transformation/transformers/es6/properties.shorthand.js b/lib/6to5/transformation/transformers/es6/properties.shorthand.js
index 9c4230fdeb..81fd7c40cb 100644
--- a/lib/6to5/transformation/transformers/es6/properties.shorthand.js
+++ b/lib/6to5/transformation/transformers/es6/properties.shorthand.js
@@ -4,6 +4,10 @@ var nameMethod = require("../../helpers/name-method");
 var t          = require("../../../types");
 var clone      = require("lodash/lang/clone");
 
+exports.check = function (node) {
+  return t.isProperty(node) && (node.method || node.shorthand);
+};
+
 exports.Property = function (node, parent, scope, context, file) {
   if (node.method) {
     node.method = false;
diff --git a/lib/6to5/transformation/transformers/es6/spread.js b/lib/6to5/transformation/transformers/es6/spread.js
index c37e1905b3..46ffecfcae 100644
--- a/lib/6to5/transformation/transformers/es6/spread.js
+++ b/lib/6to5/transformation/transformers/es6/spread.js
@@ -1,7 +1,9 @@
 "use strict";
 
-var t = require("../../../types");
 var contains = require("lodash/collection/contains");
+var t        = require("../../../types");
+
+exports.check = t.isSpreadElement;
 
 var getSpreadLiteral = function (spread, file) {
   return file.toArray(spread.argument);
diff --git a/lib/6to5/transformation/transformers/es6/template-literals.js b/lib/6to5/transformation/transformers/es6/template-literals.js
index 1f5cd55014..4fcb9243b3 100644
--- a/lib/6to5/transformation/transformers/es6/template-literals.js
+++ b/lib/6to5/transformation/transformers/es6/template-literals.js
@@ -6,6 +6,10 @@ var buildBinaryExpression = function (left, right) {
   return t.binaryExpression("+", left, right);
 };
 
+exports.check = function (node) {
+  return t.isTemplateLiteral(node) || t.isTaggedTemplateExpression(node);
+};
+
 exports.TaggedTemplateExpression = function (node, parent, scope, context, file) {
   var args = [];
   var quasi = node.quasi;
diff --git a/lib/6to5/transformation/transformers/es6/unicode-regex.js b/lib/6to5/transformation/transformers/es6/unicode-regex.js
index bccb76303d..0f4964d243 100644
--- a/lib/6to5/transformation/transformers/es6/unicode-regex.js
+++ b/lib/6to5/transformation/transformers/es6/unicode-regex.js
@@ -2,6 +2,11 @@
 
 var rewritePattern = require("regexpu/rewrite-pattern");
 var pull           = require("lodash/array/pull");
+var t              = require("../../../types");
+
+exports.check = function (node) {
+  return t.isLiteral(node) && node.regex && node.regex.flags.indexOf("u") >= 0;
+};
 
 exports.Literal = function (node) {
   var regex = node.regex;
diff --git a/lib/6to5/transformation/transformers/internal/modules.js b/lib/6to5/transformation/transformers/internal/modules.js
index 1477c94b0f..5c0a82f2cc 100644
--- a/lib/6to5/transformation/transformers/internal/modules.js
+++ b/lib/6to5/transformation/transformers/internal/modules.js
@@ -15,6 +15,10 @@ var resolveModuleSource = function (node, parent, scope, context, file) {
   }
 };
 
+exports.check = function (node) {
+  return t.isImportDeclaration(node) || t.isExportDeclaration(node);
+};
+
 exports.ImportDeclaration = resolveModuleSource;
 
 exports.ExportDeclaration = function (node, parent, scope) {
diff --git a/lib/6to5/transformation/transformers/other/react.js b/lib/6to5/transformation/transformers/other/react.js
index a246c682db..e265fb7b1e 100644
--- a/lib/6to5/transformation/transformers/other/react.js
+++ b/lib/6to5/transformation/transformers/other/react.js
@@ -10,6 +10,12 @@ var esutils  = require("esutils");
 var react    = require("../../helpers/react");
 var t        = require("../../../types");
 
+exports.check = function (node) {
+  if (t.isJSX(node)) return true;
+  if (react.isCreateClass(node)) return true;
+  return false;
+};
+
 exports.JSXIdentifier = function (node, parent) {
   if (node.name === "this" && t.isReferenced(node, parent)) {
     return t.thisExpression();
diff --git a/lib/6to5/transformation/transformers/other/regenerator.js b/lib/6to5/transformation/transformers/other/regenerator.js
index 7199c70fb5..d4a9119e7a 100644
--- a/lib/6to5/transformation/transformers/other/regenerator.js
+++ b/lib/6to5/transformation/transformers/other/regenerator.js
@@ -1,6 +1,11 @@
 "use strict";
 
 var regenerator = require("regenerator-6to5");
+var t           = require("../../../types");
+
+exports.check = function (node) {
+  return t.isFunction(node) && (node.async || node.generator);
+};
 
 exports.ast = {
   before: function (ast, file) {
diff --git a/lib/6to5/transformation/transformers/playground/object-getter-memoization.js b/lib/6to5/transformation/transformers/playground/object-getter-memoization.js
index 07ee0dc18a..e27da96702 100644
--- a/lib/6to5/transformation/transformers/playground/object-getter-memoization.js
+++ b/lib/6to5/transformation/transformers/playground/object-getter-memoization.js
@@ -23,6 +23,7 @@ exports.Property =
 exports.MethodDefinition = function (node, parent, scope, context, file) {
   if (node.kind !== "memo") return;
   node.kind = "get";
+  file.checkNode(node, scope);
 
   var value = node.value;
   t.ensureBlock(value);
diff --git a/lib/6to5/traverse/index.js b/lib/6to5/traverse/index.js
index 7279bad14d..e376908fe1 100644
--- a/lib/6to5/traverse/index.js
+++ b/lib/6to5/traverse/index.js
@@ -10,11 +10,12 @@ var contains = require("lodash/collection/contains");
 var flatten  = require("lodash/array/flatten");
 var compact  = require("lodash/array/compact");
 
-function TraversalContext() {
+function TraversalContext(scope) {
   this.shouldFlatten = false;
   this.shouldRemove  = false;
   this.shouldSkip    = false;
   this.shouldStop    = false;
+  this.scope         = scope;
 }
 
 TraversalContext.prototype.flatten = function () {
@@ -44,20 +45,31 @@ TraversalContext.prototype.reset = function () {
 TraversalContext.prototype.maybeRemove = function (obj, key) {
   if (this.shouldRemove) {
     obj[key] = null;
-    this.shouldFlatten = true;
+    this.flatten();
   }
 };
 
-function replaceNode(obj, key, node, result) {
-  var isArray = Array.isArray(result);
+TraversalContext.prototype.replaceNode = function (obj, key, node, replacement, scope) {
+  var isArray = Array.isArray(replacement);
 
   // inherit comments from original node to the first replacement node
-  var inheritTo = result;
-  if (isArray) inheritTo = result[0];
+  var inheritTo = replacement;
+  if (isArray) inheritTo = replacement[0];
   if (inheritTo) t.inheritsComments(inheritTo, node);
 
   // replace the node
-  obj[key] = result;
+  obj[key] = replacement;
+
+  var file = this.scope && this.scope.file;
+  if (file) {
+    if (isArray) {
+      for (var i = 0; i < replacement.length; i++) {
+        file.checkNode(replacement[i], scope);
+      }
+    } else {
+      file.checkNode(replacement, scope);
+    }
+  }
 
   // we're replacing a statement or block node with an array of statements so we better
   // ensure that it's a block
@@ -66,39 +78,16 @@ function replaceNode(obj, key, node, result) {
   }
 
   if (isArray) {
-    return true;
+    this.flatten();
   }
-}
-
-TraversalContext.prototype.enterNode = function (obj, key, node, enter, parent, scope, state) {
-  var result = enter(node, parent, scope, this, state);
-  var flatten = false;
-
-  if (result) {
-    flatten = replaceNode(obj, key, node, result);
-    node = result;
-
-    if (flatten) {
-      this.shouldFlatten = true;
-    }
-  }
-
-  this.maybeRemove(obj, key);
-
-  return node;
 };
 
-TraversalContext.prototype.exitNode = function (obj, key, node, exit, parent, scope, state) {
-  var result = exit(node, parent, scope, this, state);
-  var flatten = false;
+TraversalContext.prototype.call = function (fn, obj, key, node, parent, scope, state) {
+  var replacement = fn(node, parent, scope, this, state);
 
-  if (result) {
-    flatten = replaceNode(obj, key, node, result);
-    node = result;
-
-    if (flatten) {
-      this.shouldFlatten = true;
-    }
+  if (replacement) {
+    this.replaceNode(obj, key, node, replacement, scope);
+    node = replacement;
   }
 
   this.maybeRemove(obj, key);
@@ -122,7 +111,7 @@ TraversalContext.prototype.visitNode = function (obj, key, opts, scope, parent,
     ourScope = new Scope(node, parent, scope);
   }
 
-  node = this.enterNode(obj, key, node, opts.enter, parent, ourScope, state);
+  node = this.call(opts.enter, obj, key, node, parent, ourScope, state);
 
   if (this.shouldSkip) {
     return this.shouldStop;
@@ -136,7 +125,7 @@ TraversalContext.prototype.visitNode = function (obj, key, opts, scope, parent,
     }
   } else {
     traverseNode(node, opts, ourScope, state);
-    this.exitNode(obj, key, node, opts.exit, parent, ourScope, state);
+    this.call(opts.exit, obj, key, node, parent, ourScope, state);
   }
 
   return this.shouldStop;
@@ -174,7 +163,7 @@ function traverseNode(node, opts, scope, state) {
   var keys = t.VISITOR_KEYS[node.type];
   if (!keys) return;
 
-  var context = new TraversalContext();
+  var context = new TraversalContext(scope);
   for (var i = 0; i < keys.length; i++) {
     if (context.visit(node, keys[i], opts, scope, state)) {
       return;
diff --git a/lib/6to5/traverse/scope.js b/lib/6to5/traverse/scope.js
index ca04afd3b3..1f05f05b86 100644
--- a/lib/6to5/traverse/scope.js
+++ b/lib/6to5/traverse/scope.js
@@ -35,6 +35,18 @@ function Scope(block, parentBlock, parent, file) {
 
 Scope.defaultDeclarations = flatten([globals.builtin, globals.browser, globals.node].map(Object.keys));
 
+/**
+ * Description
+ *
+ * @param {Object} node
+ * @param {Object} opts
+ * @param [state]
+ */
+
+Scope.prototype.traverse = function (node, opts, state) {
+  traverse(node, opts, this, state);
+};
+
 /**
  * Description
  *
diff --git a/lib/6to5/types/alias-keys.json b/lib/6to5/types/alias-keys.json
index 5fae4406da..52ae624e72 100644
--- a/lib/6to5/types/alias-keys.json
+++ b/lib/6to5/types/alias-keys.json
@@ -44,28 +44,39 @@
   "ArrayPattern":      ["Pattern"],
   "AssignmentPattern": ["Pattern"],
 
-  "Property": ["UserWhitespacable"],
+  "Property":   ["UserWhitespacable"],
   "JSXElement": ["UserWhitespacable", "Expression"],
 
-  "ArrayExpression": ["Expression"],
-  "AssignmentExpression": ["Expression"],
-  "AwaitExpression": ["Expression"],
-  "BindFunctionExpression": ["Expression"],
-  "BindMemberExpression": ["Expression"],
-  "CallExpression": ["Expression"],
-  "ComprehensionExpression": ["Expression", "Scope"],
-  "ConditionalExpression": ["Expression"],
-  "Identifier": ["Expression"],
-  "Literal": ["Expression"],
-  "MemberExpression": ["Expression"],
-  "NewExpression": ["Expression"],
-  "ObjectExpression": ["Expression"],
-  "SequenceExpression": ["Expression"],
-  "TaggedTemplateExpression": ["Expression"],
-  "ThisExpression": ["Expression"],
-  "UpdateExpression": ["Expression"],
+  "ArrayExpression":           ["Expression"],
+  "AssignmentExpression":      ["Expression"],
+  "AwaitExpression":           ["Expression"],
+  "BindFunctionExpression":    ["Expression"],
+  "BindMemberExpression":      ["Expression"],
+  "CallExpression":            ["Expression"],
+  "ComprehensionExpression":   ["Expression", "Scope"],
+  "ConditionalExpression":     ["Expression"],
+  "Identifier":                ["Expression"],
+  "Literal":                   ["Expression"],
+  "MemberExpression":          ["Expression"],
+  "NewExpression":             ["Expression"],
+  "ObjectExpression":          ["Expression"],
+  "SequenceExpression":        ["Expression"],
+  "TaggedTemplateExpression":  ["Expression"],
+  "ThisExpression":            ["Expression"],
+  "UpdateExpression":          ["Expression"],
   "VirtualPropertyExpression": ["Expression"],
-  "JSXEmptyExpression": ["Expression"],
-  "JSXMemberExpression": ["Expression"],
-  "YieldExpression": ["Expression"]
+  "JSXEmptyExpression":        ["Expression"],
+  "JSXMemberExpression":       ["Expression"],
+  "YieldExpression":           ["Expression"],
+
+  "JSXAttribute":           ["JSX"],
+  "JSXClosingElement":      ["JSX"],
+  "JSXElement":             ["JSX"],
+  "JSXEmptyExpression":     ["JSX"],
+  "JSXExpressionContainer": ["JSX"],
+  "JSXIdentifier":          ["JSX"],
+  "JSXMemberExpression":    ["JSX"],
+  "JSXNamespacedName":      ["JSX"],
+  "JSXOpeningElement":      ["JSX"],
+  "JSXSpreadAttribute":     ["JSX"]
 }
diff --git a/lib/6to5/util.js b/lib/6to5/util.js
index 49b381ef29..8cf3aa3fd7 100644
--- a/lib/6to5/util.js
+++ b/lib/6to5/util.js
@@ -138,7 +138,7 @@ exports.parse = function (opts, code, callback) {
 
     var ast = acorn.parse(code, {
       allowImportExportEverywhere: opts.allowImportExportEverywhere,
-      allowReturnOutsideFunction:  true,
+      allowReturnOutsideFunction:  !opts._anal,
       ecmaVersion:                 opts.experimental ? 7 : 6,
       playground:                  opts.playground,
       strictMode:                  opts.strictMode,
diff --git a/test/_helper.js b/test/_helper.js
index bb8c7c3341..c44d9f672f 100644
--- a/test/_helper.js
+++ b/test/_helper.js
@@ -1,7 +1,8 @@
-var util = require("../lib/6to5/util");
-var path = require("path");
-var fs   = require("fs");
-var _    = require("lodash");
+var esvalid = require("esvalid");
+var util    = require("../lib/6to5/util");
+var path    = require("path");
+var fs      = require("fs");
+var _       = require("lodash");
 
 var humanize = function (val, noext) {
   if (noext) val = path.basename(val, path.extname(val));
@@ -18,6 +19,17 @@ var readFile = exports.readFile = function (filename) {
   }
 };
 
+exports.esvalid = function (ast, loc) {
+  var errors = esvalid.errors(ast);
+  if (errors.length) {
+    var msg = [];
+    _.each(errors, function (err) {
+      msg.push(err.message + " - " + JSON.stringify(err.node));
+    });
+    throw new Error(loc + ": " + msg.join(". "));
+  }
+};
+
 exports.assertVendor = function (name) {
   if (!fs.existsSync(__dirname + "/../vendor/" + name)) {
     console.error("No vendor/" + name + " - run `make bootstrap`");
diff --git a/test/_transformation-helper.js b/test/_transformation-helper.js
index 09535e04f3..3d6fb56166 100644
--- a/test/_transformation-helper.js
+++ b/test/_transformation-helper.js
@@ -2,7 +2,6 @@ var genHelpers = require("./_generator-helpers");
 var transform  = require("../lib/6to5/transformation");
 var sourceMap  = require("source-map");
 var codeFrame  = require("../lib/6to5/helpers/code-frame");
-var esvalid    = require("esvalid");
 var Module     = require("module");
 var helper     = require("./_helper");
 var assert     = require("assert");
@@ -55,15 +54,7 @@ var run = function (task, done) {
 
   var checkAst = function (result, opts) {
     if (noCheckAst) return;
-
-    var errors = esvalid.errors(result.ast.program);
-    if (errors.length) {
-      var msg = [];
-      _.each(errors, function (err) {
-        msg.push(err.message + " - " + JSON.stringify(err.node));
-      });
-      throw new Error(opts.loc + ": " + msg.join(". "));
-    }
+    helper.esvalid(result.ast.program, opts.loc);
   };
 
   if (execCode) {
diff --git a/test/test262.js b/test/test262.js
index 11c96abcfe..e77d04f638 100644
--- a/test/test262.js
+++ b/test/test262.js
@@ -4,6 +4,7 @@ require("./_helper").assertVendor("test262");
 
 var transform = require("../lib/6to5/transformation");
 var readdir   = require("fs-readdir-recursive");
+var helper    = require("./_helper");
 var path      = require("path");
 var fs        = require("fs");
 var _         = require("lodash");
@@ -16,7 +17,7 @@ var read = function (loc) {
   });
 };
 
-var exec = function (loc) {
+var check = function (loc) {
   try {
     var file = fs.readFileSync(loc, "utf8");
 
@@ -26,25 +27,21 @@ var exec = function (loc) {
     // ReferenceError: 1++; (runtime)
     var lazyError = /negative: (\S+)/.test(file);
 
-    var compiled = transform(file, {
+    transform(file, {
       filename: loc,
-      blacklist: ["useStrict"]
+      blacklist: ["useStrict"],
+      _anal: true
     });
-
-    global.eval(compiled);
   } catch (err) {
     if (err && lazyError && err instanceof SyntaxError) {
       return;
     } else {
+      err.stack = loc + ": " + err.stack;
       throw err;
     }
   }
 };
 
-// harness
-var harness = read(test262Loc + "/harness");
-_.each(harness, exec);
-
 // tests!
 var tests = read(test262Loc + "/test");
 _.each(tests, function (loc) {
@@ -52,6 +49,6 @@ _.each(tests, function (loc) {
   alias = alias.replace(/\.([^\.]+)$/g, "");
   test(alias, function () {
     this.timeout(0);
-    exec(loc);
+    check(loc);
   });
 });