get rid of _let

packages/babel-plugin-transform-es2015-block-scoping/src/index.js

@@ -71,7 +71,7 @@ let buildRetCheck = template(`
 
 function isBlockScoped(node) {
   if (!t.isVariableDeclaration(node)) return false;
-  if (node._let) return true;
+  if (node[t.BLOCK_SCOPED_SYMBOL]) return true;
   if (node.kind !== "let" && node.kind !== "const") return false;
   return true;
 }
@@ -85,18 +85,13 @@ function convertBlockScopedToVar(node, parent, scope) {
     }
   }
 
+  node[t.BLOCK_SCOPED_SYMBOL] = true;
   node._let = true;
   node.kind = "var";
 }
 
-function isVar(node, parent) {
-  return t.isVariableDeclaration(node, { kind: "var" }) && !isBlockScoped(node, parent);
-}
-
-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) {
@@ -527,9 +522,6 @@ 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,

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];
   }

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("var used to be block scoped");

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];
 }
 
 /**