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..3f8ef5a75c8b0392fd623e772230ea5021108e11 100644
--- a/sdk/lib/_internal/compiler/js_lib/collection_patch.dart
+++ b/sdk/lib/_internal/compiler/js_lib/collection_patch.dart
@@ -386,6 +386,7 @@ 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 +493,7 @@ class LinkedHashMap<K, V> {
     if (isValidKey == null) {
       if (hashCode == null) {
         if (equals == null) {
-          return new JsLinkedHashMap<K, V>();
+          return new JsLinkedHashMap<K, V>.es6();
         }
         hashCode = _defaultHashCode;
       } else {
@@ -521,16 +522,17 @@ class LinkedHashMap<K, V> {
   // Private factory constructor called by generated code for map literals.
   @NoInline()
   factory LinkedHashMap._literal(List keyValuePairs) {
-    return fillLiteralMap(keyValuePairs, new JsLinkedHashMap<K, V>());
+    return fillLiteralMap(keyValuePairs, new JsLinkedHashMap<K, V>.es6());
   }
 
   // Private factory constructor called by generated code for map literals.
   @NoThrows() @NoInline()
   factory LinkedHashMap._empty() {
-    return new JsLinkedHashMap<K, V>();
+    return new JsLinkedHashMap<K, V>.es6();
   }
 }
 
+// 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
@@ -550,10 +552,12 @@ 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);