Replace Backend with JavaScriptBackend.

pkg/compiler/lib/src/common/codegen.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.common.codegen;
 
 import '../common.dart';
-import '../common/backend_api.dart' show Backend;
 import '../constants/values.dart' show ConstantValue;
 import '../elements/resolution_types.dart'
     show ResolutionDartType, ResolutionInterfaceType;
 import '../elements/elements.dart'
     show
         AstElement,
         ClassElement,
         Element,
         FunctionElement,
         LocalFunctionElement,
         ResolvedAst;
+import '../js_backend/backend.dart' show JavaScriptBackend;
 import '../universe/use.dart' show DynamicUse, StaticUse, TypeUse;
 import '../universe/world_impact.dart'
     show WorldImpact, WorldImpactBuilderImpl, WorldImpactVisitor;
 import '../util/util.dart' show Pair, Setlet;
 import 'work.dart' show WorkItem;
 
 class CodegenImpact extends WorldImpact {
   const CodegenImpact();
 
   Iterable<ConstantValue> get compileTimeConstants => const <ConstantValue>[];
@@ skipping 188 matching lines @@
 
   void registerAsyncMarker(FunctionElement element) {
     worldImpact.registerAsyncMarker(element);
   }
 }
 
 /// [WorkItem] used exclusively by the [CodegenEnqueuer].
 class CodegenWorkItem extends WorkItem {
   CodegenRegistry registry;
   final ResolvedAst resolvedAst;
-  final Backend backend;
+  final JavaScriptBackend backend;
 
-  factory CodegenWorkItem(Backend backend, AstElement element) {
+  factory CodegenWorkItem(JavaScriptBackend backend, AstElement element) {
     // If this assertion fails, the resolution callbacks of the backend may be
     // missing call of form registry.registerXXX. Alternatively, the code
     // generation could spuriously be adding dependencies on things we know we
     // don't need.
     assert(invariant(element, element.hasResolvedAst,
         message: "$element has no resolved ast."));
     ResolvedAst resolvedAst = element.resolvedAst;
     return new CodegenWorkItem.internal(resolvedAst, backend);
   }
 
   CodegenWorkItem.internal(ResolvedAst resolvedAst, this.backend)
       : this.resolvedAst = resolvedAst,
         super(resolvedAst.element);
 
   WorldImpact run() {
     registry = new CodegenRegistry(element);
     return backend.codegen(this);
   }
 
   String toString() => 'CodegenWorkItem(${resolvedAst.element})';
 }