Make EfficientLength public, as EfficientLengthIterable.

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;
 
   bool get isNotEmpty => !isEmpty;
 
   String toString() => Maps.mapToString(this);
 
   _throwUnmodifiable() {
     throw new UnsupportedError("Cannot modify unmodifiable Map");
   }
   void operator []=(K key, V val) => _throwUnmodifiable();
   V putIfAbsent(K key, V ifAbsent()) => _throwUnmodifiable();
   V remove(K key) => _throwUnmodifiable();
   void clear() => _throwUnmodifiable();
   void addAll(Map<K, V> other) => _throwUnmodifiable();
 }
 
-class ConstantStringMap<K, V> extends ConstantMap<K, V>
-                              implements _symbol_dev.EfficientLength {
+class ConstantStringMap<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.
   const ConstantStringMap._(this.length, this._jsObject, this._keys)
       : super._();
 
   final int length;
   // A constant map is backed by a JavaScript object.
   final _jsObject;
   final List<K> _keys;
@@ skipping 102 matching lines @@
   Iterable<K> get keys {
     return _getMap().keys;
   }
 
   Iterable<V> get values {
     return _getMap().values;
   }
 
   int get length => _getMap().length;
 }