Make [length] efficient for [ConstantMap.keys].

sdk/lib/_internal/compiler/js_lib/constant_map.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.
 
 part of _js_helper;
 
 abstract class ConstantMap<K, V> implements Map<K, V> {
   const ConstantMap._();
 
   bool get isEmpty => length == 0;
@@ skipping 79 matching lines @@
 
   _fetch(key) =>
       '__proto__' == key ? _protoValue : jsPropertyAccess(_jsObject, key);
 }
 
 class _ConstantMapKeyIterable<K> extends IterableBase<K> {
   ConstantStringMap<K, dynamic> _map;
   _ConstantMapKeyIterable(this._map);
 
   Iterator<K> get iterator => _map._keys.iterator;
+
+  int get length => _map._keys.length;
 }
 
 class GeneralConstantMap<K, V> extends ConstantMap<K, V> {
   // This constructor is not used.  The instantiation is shortcut by the
   // compiler. It is here to make the uninitialized final fields legal.
   GeneralConstantMap(this._jsData) : super._();
 
   // [_jsData] holds a key-value pair list.
   final _jsData;
 
@@ skipping 26 matching lines @@
   Iterable<K> get keys {
     return _getMap().keys;
   }
 
   Iterable<V> get values {
     return _getMap().values;
   }
 
   int get length => _getMap().length;
 }