Add EfficientLength marker interface to some iterabels.

sdk/lib/core/iterable.dart

Index: sdk/lib/core/iterable.dart
diff --git a/sdk/lib/core/iterable.dart b/sdk/lib/core/iterable.dart
index 0e1f05a123f0839573de7176c7bf1eca331a681c..42ecd46652decb7d73fd399d9ac41332770d25e3 100644
--- a/sdk/lib/core/iterable.dart
+++ b/sdk/lib/core/iterable.dart
@@ -201,7 +201,7 @@ abstract class Iterable<E> {
   /**
    * 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.
@@ -280,14 +280,15 @@ abstract class Iterable<E> {
   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> {