Add EfficientLength marker interface to some iterabels.

runtime/lib/collection_patch.dart

Index: runtime/lib/collection_patch.dart
diff --git a/runtime/lib/collection_patch.dart b/runtime/lib/collection_patch.dart
index 9f0c74fad4a7efd0bc94e22c91b32e61cbf21786..6785064b4c1ab749829050f345f877b22355f454 100644
--- a/runtime/lib/collection_patch.dart
+++ b/runtime/lib/collection_patch.dart
@@ -422,7 +422,8 @@ class _HashMapEntry {
   _HashMapEntry(this.key, this.value, this.hashCode, this.next);
 }
 
-abstract class _HashMapIterable<E> extends IterableBase<E> {
+abstract class _HashMapIterable<E> extends IterableBase<E>
+                                   implements EfficientLength {
   final HashMap _map;
   _HashMapIterable(this._map);
   int get length => _map.length;
@@ -812,7 +813,8 @@ class _LinkedHashMapEntry extends _HashMapEntry {
   }
 }
 
-class _LinkedHashMapKeyIterable<K> extends IterableBase<K> {
+class _LinkedHashMapKeyIterable<K> extends IterableBase<K>
+                                   implements EfficientLength {
   LinkedHashMap<K, dynamic> _map;
   _LinkedHashMapKeyIterable(this._map);
   Iterator<K> get iterator => new _LinkedHashMapKeyIterator<K>(_map);
@@ -822,7 +824,8 @@ class _LinkedHashMapKeyIterable<K> extends IterableBase<K> {
   int get length => _map.length;
 }
 
-class _LinkedHashMapValueIterable<V> extends IterableBase<V> {
+class _LinkedHashMapValueIterable<V> extends IterableBase<V>
+                                     implements EfficientLength {
   LinkedHashMap<dynamic, V> _map;
   _LinkedHashMapValueIterable(this._map);
   Iterator<K> get iterator => new _LinkedHashMapValueIterator<V>(_map);