Add SetBase/SetMixin.

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 81aa289f17ebc60e01e8982f2ca2cd7949a5843d..e0fd182cb5539e4df421b76a83635abdf9042ed5 100644
--- a/runtime/lib/collection_patch.dart
+++ b/runtime/lib/collection_patch.dart
@@ -634,10 +634,6 @@ class _HashSet<E> extends _HashSetBase<E> implements HashSet<E> {
     }
   }
 
-  void retainAll(Iterable<Object> objectsToRetain) {
-    super._retainAll(objectsToRetain, (o) => o is E);
-  }
-
   void _filterWhere(bool test(E element), bool removeMatching) {
     int length = _buckets.length;
     for (int index =  0; index < length; index++) {
@@ -757,10 +753,6 @@ class _CustomHashSet<E> extends _HashSet<E> {
     }
   }
 
-  void retainAll(Iterable<Object> elements) {
-    super._retainAll(elements, _validKey);
-  }
-
   bool _equals(e1, e2) => _equality(e1, e2);
   int _hashCode(e) => _hasher(e);
 
@@ -1225,10 +1217,6 @@ class _LinkedCustomHashSet<E> extends _LinkedHashSet<E> {
     }
   }
 
-  void retainAll(Iterable<Object> elements) {
-    super._retainAll(elements, _validKey);
-  }
-
   HashSet<E> _newSet() =>
       new _LinkedCustomHashSet<E>(_equality, _hasher, _validKey);
 }