Make EfficientLength public, as EfficientLengthIterable.

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 2377c8c3d0909c964b6828498209839abc9f91b6..b4fcbcfdefb8987c5853048844194e9069721385 100644
--- a/runtime/lib/immutable_map.dart
+++ b/runtime/lib/immutable_map.dart
@@ -84,8 +84,8 @@ class ImmutableMap<K, V> implements Map<K, V> {
   }
 }
 
-class _ImmutableMapKeyIterable<E> extends IterableBase<E>
-                                  implements EfficientLength {
+class _ImmutableMapKeyIterable<E> extends Iterable<E>
+                                  implements EfficientLengthIterable<E> {
   final ImmutableMap _map;
   _ImmutableMapKeyIterable(this._map);
 
@@ -96,8 +96,8 @@ class _ImmutableMapKeyIterable<E> extends IterableBase<E>
   int get length => _map.length;
 }
 
-class _ImmutableMapValueIterable<E> extends IterableBase<E>
-                                    implements EfficientLength {
+class _ImmutableMapValueIterable<E> extends Iterable<E>
+                                    implements EfficientLengthIterable<E> {
   final ImmutableMap _map;
   _ImmutableMapValueIterable(this._map);