Reduce use of Compiler.closedWorld

pkg/compiler/lib/src/common/backend_api.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.backend_api;
 
 import 'dart:async' show Future;
 
 import '../common.dart';
 import '../common/codegen.dart' show CodegenImpact;
@@ skipping 379 matching lines @@
   DeserializerPlugin get deserializer => const DeserializerPlugin();
 }
 
 /// Interface providing access to core classes used by the backend.
 abstract class BackendClasses {
   ClassElement get intImplementation;
   ClassElement get doubleImplementation;
   ClassElement get numImplementation;
   ClassElement get stringImplementation;
   ClassElement get listImplementation;
+  ClassElement get mutableListImplementation;
   ClassElement get growableListImplementation;
   ClassElement get fixedListImplementation;
   ClassElement get constListImplementation;
   ClassElement get mapImplementation;
   ClassElement get constMapImplementation;
   ClassElement get functionImplementation;
   ClassElement get typeImplementation;
   ClassElement get boolImplementation;
   ClassElement get nullImplementation;
   ClassElement get uint32Implementation;
   ClassElement get uint31Implementation;
   ClassElement get positiveIntImplementation;
   ClassElement get syncStarIterableImplementation;
   ClassElement get asyncFutureImplementation;
   ClassElement get asyncStarStreamImplementation;
+  ClassElement get indexableImplementation;
+  ClassElement get mutableIndexableImplementation;
+
+  bool isDefaultEqualityImplementation(Element element);
 }