Add EfficientLength marker interface to some iterabels.

sdk/lib/core/map.dart

 // Copyright (c) 2011, 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 dart.core;
 
 /**
  * An unordered collection of key-value pairs, from which you retrieve a value
  * by using its associated key.
  *
@@ skipping 135 matching lines @@
 
   /**
    * Applies [f] to each {key, value} pair of the map.
    *
    * It is an error to add or remove keys from the map during iteration.
    */
   void forEach(void f(K key, V value));
 
   /**
    * The keys of [this].
+   *
+   * The `contains` method of the returned iterable is as efficient as
+   * [containsKey].
+   *
+   * The returned iterable is guaranteed to have the same `length` as this map,
+   * and its `length` getter is efficient.
    */
   Iterable<K> get keys;
 
   /**
    * The values of [this].
+   *
+   * This iterable is guaranteed to have the same `length` as this map,
+   * and its `length` getter is efficient.
    */
   Iterable<V> get values;
 
   /**
    * The number of {key, value} pairs in the map.
+   *
+   * This operation is efficient, and doesn't require iterating and counting
+   * the entries.
    */
   int get length;
 
   /**
    * Returns true if there is no {key, value} pair in the map.
    */
   bool get isEmpty;
 
   /**
    * Returns true if there is at least one {key, value} pair in the map.
    */
   bool get isNotEmpty;
 }