Make EfficientLength extend Iterable.

runtime/lib/immutable_map.dart

Index: runtime/lib/immutable_map.dart
diff --git a/runtime/lib/immutable_map.dart b/runtime/lib/immutable_map.dart
index b8d0d3c62d7b9cb8ed74402db42413aba5f6c39e..ba866e39cf3769864345229f65abca18e3b5ae32 100644
--- a/runtime/lib/immutable_map.dart
+++ b/runtime/lib/immutable_map.dart
@@ -85,8 +85,7 @@ class ImmutableMap<K, V> implements Map<K, V> {
   }
 }
 
-class _ImmutableMapKeyIterable<E> extends IterableBase<E>
-                                  implements EfficientLength {
+class _ImmutableMapKeyIterable<E> extends EfficientLengthIterable<E> {
   final ImmutableMap _map;
   _ImmutableMapKeyIterable(this._map);
 
@@ -97,8 +96,7 @@ class _ImmutableMapKeyIterable<E> extends IterableBase<E>
   int get length => _map.length;
 }
 
-class _ImmutableMapValueIterable<E> extends IterableBase<E>
-                                    implements EfficientLength {
+class _ImmutableMapValueIterable<E> extends EfficientLengthIterable<E> {
   final ImmutableMap _map;
   _ImmutableMapValueIterable(this._map);