Add EfficientLength marker interface to some iterabels.

sdk/lib/core/list.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 dart.core;
 
 /**
  * An indexable collection of objects with a length.
  *
  * Subclasses of this class implement different kinds of lists.
@@ skipping 18 matching lines @@
  *     growableList[0] = 87;
  *
  * Lists are [Iterable]. Iteration occurs over values in index order. Changing
  * the values does not affect iteration, but changing the valid
  * indices—that is, changing the list's length—between iteration
  * steps causes a [ConcurrentModificationError]. This means that only growable
  * lists can throw ConcurrentModificationError. If the length changes
  * temporarily and is restored before continuing the iteration, the iterator
  * does not detect it.
  */
-abstract class List<E> implements Iterable<E> {
+abstract class List<E> implements Iterable<E>, EfficientLength {
   /**
    * Creates a list of the given length.
    *
    * The created list is fixed-length if [length] is provided.
    *
    *     List fixedLengthList = new List(3);
    *     fixedLengthList.length;     // 3
          fixedLengthList.length = 1; // Error
    *
    *
@@ skipping 71 matching lines @@
   /**
    * Sets the value at the given [index] in the list to [value]
    * or throws a [RangeError] if [index] is out of bounds.
    */
   void operator []=(int index, E value);
 
   /**
    * Returns the number of objects in this list.
    *
    * The valid indices for a list are `0` through `length - 1`.
+   *
+   * This operations is efficient, and does not require iterating and counting
+   * the elements.
    */
   int get length;
 
   /**
    * Changes the length of this list.
    *
    * If [newLength] is greater than
    * the current length, entries are initialized to [:null:].
    *
    * Throws an [UnsupportedError] if the list is fixed-length.
@@ skipping 288 matching lines @@
    * as values. The `Map.keys` [Iterable] iterates the indices of this list
    * in numerical order.
    *
    *     List<String> words = ['fee', 'fi', 'fo', 'fum'];
    *     Map<int, String> map = words.asMap();
    *     map[0] + map[1];   // 'feefi';
    *     map.keys.toList(); // [0, 1, 2, 3]
    */
   Map<int, E> asMap();
 }