dart2js: use Es6 maps when available.

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

Index: sdk/lib/_internal/compiler/js_lib/collection_patch.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/collection_patch.dart b/sdk/lib/_internal/compiler/js_lib/collection_patch.dart
index 67e54bc0978c6367684ee3b35025e006361add9f..509b83edd0cb82569cb54c94dea5f04db478de5e 100644
--- a/sdk/lib/_internal/compiler/js_lib/collection_patch.dart
+++ b/sdk/lib/_internal/compiler/js_lib/collection_patch.dart
@@ -5,7 +5,8 @@
 // Patch file for dart:collection classes.
 import 'dart:_foreign_helper' show JS;
 import 'dart:_js_helper' show
-    fillLiteralMap, InternalMap, NoInline, NoThrows, patch, JsLinkedHashMap,
+    fillLiteralMap, InternalMap, NoInline, NoThrows, patch,
+    JsLinkedHashMap, Es6LinkedHashMap,
     LinkedHashMapCell, LinkedHashMapKeyIterable, LinkedHashMapKeyIterator;
 
 @patch
@@ -386,6 +387,7 @@ class _CustomHashMap<K, V> extends _HashMap<K, V> {
   final _Equality<K> _equals;
   final _Hasher<K> _hashCode;
   final _Predicate _validKey;
+
   _CustomHashMap(this._equals, this._hashCode, bool validKey(potentialKey))
       : _validKey = (validKey != null) ? validKey : ((v) => v is K);
 
@@ -492,7 +494,11 @@ class LinkedHashMap<K, V> {
     if (isValidKey == null) {
       if (hashCode == null) {
         if (equals == null) {
-          return new JsLinkedHashMap<K, V>();
+          if (JsLinkedHashMap.supportsEs6Maps) {
+            return new Es6LinkedHashMap<K, V>();
+          } else {
+            return new JsLinkedHashMap<K, V>();
+          }
         }
         hashCode = _defaultHashCode;
       } else {
@@ -531,7 +537,9 @@ class LinkedHashMap<K, V> {
   }
 }
 
+// TODO(floitsch): use ES6 Maps when available.
 class _LinkedIdentityHashMap<K, V> extends JsLinkedHashMap<K, V> {
+
   int internalComputeHashCode(var key) {
     // We force the hash codes to be unsigned 30-bit integers to avoid
     // issues with problematic keys like '__proto__'. Another option
@@ -550,14 +558,17 @@ class _LinkedIdentityHashMap<K, V> extends JsLinkedHashMap<K, V> {
   }
 }
 
+// TODO(floitsch): use ES6 maps when available.
 class _LinkedCustomHashMap<K, V> extends JsLinkedHashMap<K, V> {
   final _Equality<K> _equals;
   final _Hasher<K> _hashCode;
   final _Predicate _validKey;
+
   _LinkedCustomHashMap(this._equals, this._hashCode,
                        bool validKey(potentialKey))
       : _validKey = (validKey != null) ? validKey : ((v) => v is K);
 
+
   V operator[](Object key) {
     if (!_validKey(key)) return null;
     return super.internalGet(key);