Add VM class for Map/LinkedHashMap.

runtime/lib/collection_patch.dart

 // Copyright (c) 2013, 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.
 
 patch class HashMap<K, V> {
   /* patch */ factory HashMap({ bool equals(K key1, K key2),
                                 int hashCode(K key),
                                 bool isValidKey(potentialKey) }) {
     if (isValidKey == null) {
       if (hashCode == null) {
@@ skipping 888 matching lines @@
   /// only the head entry itself).
   var _nextEntry;
   var _previousEntry;
 
   /* patch */ factory LinkedHashMap({ bool equals(K key1, K key2),
                                       int hashCode(K key),
                                       bool isValidKey(potentialKey) }) {
     if (isValidKey == null) {
       if (hashCode == null) {
         if (equals == null) {
-          return new _LinkedHashMap<K, V>();
+          if (_useInternalCached) {
+            return new _InternalLinkedHashMap<K, V>();
+          } else {
+            return new _LinkedHashMap<K, V>();
+          }
         }
         hashCode = _defaultHashCode;
       } else {
         if (identical(identityHashCode, hashCode) &&
             identical(identical, equals)) {
           return new _LinkedIdentityHashMap<K, V>();
         }
         if (equals == null) {
           equals = _defaultEquals;
         }
       }
     } else {
       if (hashCode == null) {
         hashCode = _defaultHashCode;
       }
       if (equals == null) {
         equals = _defaultEquals;
       }
     }
     return new _LinkedCustomHashMap<K, V>(equals, hashCode, isValidKey);
   }
 
   /* patch */ factory LinkedHashMap.identity() = _LinkedIdentityHashMap<K, V>;
+
+  static final bool _useInternalCached = _useInternal;
+  static bool get _useInternal native "LinkedHashMap_useInternal";
 }
 
 // Methods that are exactly the same in all three linked hash map variants.
 abstract class _LinkedHashMapMixin<K, V> implements LinkedHashMap<K, V> {
   var _nextEntry;
   var _previousEntry;
 
 
   bool containsValue(Object value) {
     int modificationCount = _modificationCount;
@@ skipping 303 matching lines @@
       return false;
     }
     _LinkedHashSetEntry entry = _next;
     _current = entry.key;
     _next = entry._nextEntry;
     return true;
   }
 
   E get current => _current;
 }