Move js library back into compiler directory.

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 125 matching lines @@
   Iterable<K> get keys {
     return _getMap().keys;
   }
 
   Iterable<V> get values {
     return _getMap().values;
   }
 
   int get length => _getMap().length;
 }