Revert "Make EfficientLength extend Iterable."

sdk/lib/collection/maps.dart

Index: sdk/lib/collection/maps.dart
diff --git a/sdk/lib/collection/maps.dart b/sdk/lib/collection/maps.dart
index d5b86210ad1fa8610fce6da2f5cfcbc6e84ed22a..7871b8720b1351190766c03865d7defec267e7c3 100644
--- a/sdk/lib/collection/maps.dart
+++ b/sdk/lib/collection/maps.dart
@@ -111,7 +111,8 @@ abstract class UnmodifiableMapBase<K, V> =
  * It accesses the values by iterating over the keys of the map, and using the
  * map's `operator[]` to lookup the keys.
  */
-class _MapBaseValueIterable<K, V> extends EfficientLengthIterable<V> {
+class _MapBaseValueIterable<K, V> extends Iterable<V>
+                                  implements EfficientLength {
   final Map<K, V> _map;
   _MapBaseValueIterable(this._map);