Move implementation from IterableBase to Iterable.

sdk/lib/_internal/compiler/js_lib/constant_map.dart

Index: sdk/lib/_internal/compiler/js_lib/constant_map.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/constant_map.dart b/sdk/lib/_internal/compiler/js_lib/constant_map.dart
index ae3e3799f33ac8eb6c77435e38ec38ef9304aac5..e716cf14bf97d1e11e686f414fac5db8dbe281e6 100644
--- a/sdk/lib/_internal/compiler/js_lib/constant_map.dart
+++ b/sdk/lib/_internal/compiler/js_lib/constant_map.dart
@@ -92,7 +92,7 @@ class ConstantProtoMap<K, V> extends ConstantStringMap<K, V> {
       '__proto__' == key ? _protoValue : jsPropertyAccess(_jsObject, key);
 }
 
-class _ConstantMapKeyIterable<K> extends IterableBase<K> {
+class _ConstantMapKeyIterable<K> extends Iterable<K> {
   ConstantStringMap<K, dynamic> _map;
   _ConstantMapKeyIterable(this._map);