Add EfficientLength marker interface to some iterabels.

runtime/lib/immutable_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.
 // Immutable map class for compiler generated map literals.
 
 class ImmutableMap<K, V> implements Map<K, V> {
   final _ImmutableList _kvPairs;
 
   const ImmutableMap._create(_ImmutableList keyValuePairs)
       : _kvPairs = keyValuePairs;
@@ skipping 66 matching lines @@
 
   V remove(Object key) {
     throw new UnsupportedError("Cannot remove from unmodifiable Map");
   }
 
   String toString() {
     return Maps.mapToString(this);
   }
 }
 
-class _ImmutableMapKeyIterable<E> extends IterableBase<E> {
+class _ImmutableMapKeyIterable<E> extends IterableBase<E>
+                                  implements EfficientLength {
   final ImmutableMap _map;
   _ImmutableMapKeyIterable(this._map);
 
   Iterator<E> get iterator {
     return new _ImmutableMapKeyIterator<E>(_map);
   }
+
+  int get length => _map.length;
 }
 
-class _ImmutableMapValueIterable<E> extends IterableBase<E> {
+class _ImmutableMapValueIterable<E> extends IterableBase<E>
+                                    implements EfficientLength {
   final ImmutableMap _map;
   _ImmutableMapValueIterable(this._map);
 
   Iterator<E> get iterator {
     return new _ImmutableMapValueIterator<E>(_map);
   }
+
+  int get length => _map.length;
 }
 
 class _ImmutableMapKeyIterator<E> implements Iterator<E> {
   ImmutableMap _map;
   int _index = -1;
   E _current;
 
   _ImmutableMapKeyIterator(this._map);
 
   bool moveNext() {
@@ skipping 25 matching lines @@
       _current = _map._kvPairs[newIndex * 2 + 1];
       return true;
     }
     _current = null;
     _index = _map.length;
     return false;
   }
 
   E get current => _current;
 }