Add EfficientLength marker interface to some iterabels.

sdk/lib/core/iterable.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 object that uses an [Iterator] to serve objects one at a time.
  *
  * You can iterate over all objects served by an Iterable object
@@ skipping 183 matching lines @@
    *
    * When the iterator encounters an element `e` that does not satisfy [test],
    * it discards `e` and moves into the finished state. That is, it does not
    * get or provide any more elements.
    */
   Iterable<E> takeWhile(bool test(E value));
 
   /**
    * Returns an Iterable that skips the first [n] elements.
    *
-   * If `this` has fewer than [n] elements, then the resulting Iterable is 
+   * If `this` has fewer than [n] elements, then the resulting Iterable is
    * empty.
    *
    * It is an error if [n] is negative.
    */
   Iterable<E> skip(int n);
 
   /**
    * Returns an Iterable that skips elements while [test] is satisfied.
    *
    * The filtering happens lazily. Every new Iterator of the returned
@@ skipping 58 matching lines @@
    *
    * If `this` has fewer than [index] elements throws a [RangeError].
    *
    * Note: if `this` does not have a deterministic iteration order then the
    * function may simply return any element without any iteration if there are
    * at least [index] elements in `this`.
    */
   E elementAt(int index);
 }
 
-
 typedef E _Generator<E>(int index);
 
-class _GeneratorIterable<E> extends IterableBase<E> {
+class _GeneratorIterable<E> extends IterableBase<E>
+                            implements EfficientLength {
   final int _count;
   final _Generator<E> _generator;
   _GeneratorIterable(this._count, this._generator);
   Iterator<E> get iterator => new _GeneratorIterator(_count, _generator);
+  int get length => _count;
 }
 
 class _GeneratorIterator<E> implements Iterator<E> {
   final int _count;
   final _Generator<E> _generator;
   int _index = 0;
   E _current;
 
   _GeneratorIterator(this._count, this._generator);
 
@@ skipping 16 matching lines @@
  */
 abstract class BidirectionalIterator<E> implements Iterator<E> {
   /**
    * Move back to the previous element.
    *
    * Returns true and updates [current] if successful. Returns false
    * and sets [current] to null if there is no previous element.
    */
   bool movePrevious();
 }