Use an ES6 map for a linked identity hash map.

sdk/lib/_internal/compiler/js_lib/foreign_helper.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library _foreign_helper;
 
 import 'dart:_js_embedded_names' show JsGetName, JsBuiltin;
 
 /**
  * Emits a JavaScript code fragment parameterized by arguments.
@@ skipping 23 matching lines @@
  *    type in Dart that is this precise.  The Dart alternative would be `Object`
  *    or `dynamic`, but these types imply that the JS-code might also be
  *    creating instances of all the DOM types.
  *
  *    If `null` is possible, it must be specified explicitly, e.g.
  *    `"String|Null"`. [typeDescription] has several extensions to help describe
  *    the behavior more accurately.  In addition to the union type already
  *    described:
  *
  *    + `=Object` is a plain JavaScript object.  Some DOM methods return
- *       instances that have no corresponing Dart type (e.g. cross-frame
+ *       instances that have no corresponding Dart type (e.g. cross-frame
  *       documents), `=Object` can be used to describe these untyped' values.
  *
  *    + `var` (or empty string).  If the entire [typeDescription] is `var` (or
  *      empty string) then the type is `dynamic` but the code is known to not
  *      create any instances.
  *
  *   Examples:
  *
  *       // Parent window might be an opaque cross-frame window.
  *       var thing = JS('=Object|Window', '#.parent', myWindow);
@@ skipping 199 matching lines @@
 }
 
 /**
  * JavaScript string concatenation. Inputs must be Strings.  Corresponds to the
  * HStringConcat SSA instruction and may be constant-folded.
  */
 String JS_STRING_CONCAT(String a, String b) {
   // This body is unused, only here for type analysis.
   return JS('String', '# + #', a, b);
 }