Revert "Make EfficientLength public, as EfficientLengthIterable."

sdk/lib/_internal/compiler/js_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 file for dart:collection classes.
 import 'dart:_foreign_helper' show JS;
 import 'dart:_js_helper' show
     fillLiteralMap, InternalMap, NoInline, NoSideEffects, NoThrows, patch,
     JsLinkedHashMap, LinkedHashMapCell, LinkedHashMapKeyIterable,
     LinkedHashMapKeyIterator;
@@ skipping 412 matching lines @@
     for (int i = 0; i < length; i += 2) {
       if (_equals(JS('var', '#[#]', bucket, i), key)) return i;
     }
     return -1;
   }
 
   String toString() => Maps.mapToString(this);
 }
 
 class HashMapKeyIterable<E> extends Iterable<E>
-                            implements EfficientLengthIterable<E> {
+                            implements EfficientLength {
   final _map;
   HashMapKeyIterable(this._map);
 
   int get length => _map._length;
   bool get isEmpty => _map._length == 0;
 
   Iterator<E> get iterator {
     return new HashMapKeyIterator<E>(_map, _map._computeKeys());
   }
 
@@ skipping 1030 matching lines @@
     } else if (_cell == null) {
       _current = null;
       return false;
     } else {
       _current = _cell._element;
       _cell = _cell._next;
       return true;
     }
   }
 }