Replace Backend with JavaScriptBackend.

pkg/compiler/lib/src/resolution/registry.dart

 // Copyright (c) 2014, 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.resolution.registry;
 
 import '../common.dart';
-import '../common/backend_api.dart'
-    show Backend, ForeignResolver, NativeRegistry;
+import '../common/backend_api.dart' show ForeignResolver, NativeRegistry;
 import '../common/resolution.dart' show ResolutionImpact, Target;
 import '../constants/expressions.dart';
 import '../elements/resolution_types.dart';
 import '../diagnostics/source_span.dart';
 import '../elements/elements.dart';
 import '../tree/tree.dart';
 import '../universe/call_structure.dart' show CallStructure;
 import '../universe/feature.dart';
 import '../universe/selector.dart' show Selector;
 import '../universe/use.dart' show DynamicUse, StaticUse, TypeUse;
@@ skipping 412 matching lines @@
   @override
   void registerInstantiatedType(ResolutionInterfaceType type) {
     registry.registerInstantiation(type);
   }
 
   @override
   ResolutionDartType resolveTypeFromString(Node node, String typeName) {
     return visitor.resolveTypeFromString(node, typeName);
   }
 }