Split js_backend into multiple libraries

pkg/compiler/lib/src/js_backend/constant_emitter.dart

Index: pkg/compiler/lib/src/js_backend/constant_emitter.dart
diff --git a/pkg/compiler/lib/src/js_backend/constant_emitter.dart b/pkg/compiler/lib/src/js_backend/constant_emitter.dart
index 7cc8dc540177f1c468ec8e919d8b0e48968e59e3..d8db2dcf9148b8d4aae5f44a267958c862ba6772 100644
--- a/pkg/compiler/lib/src/js_backend/constant_emitter.dart
+++ b/pkg/compiler/lib/src/js_backend/constant_emitter.dart
@@ -2,7 +2,18 @@
 // 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.
 
-part of js_backend;
+import '../common.dart';
+import '../compiler.dart' show Compiler;
+import '../constants/values.dart';
+import '../dart_types.dart';
+import '../elements/elements.dart';
+import '../io/code_output.dart';
+import '../js/js.dart' as jsAst;
+import '../js/js.dart' show js;
+import 'constant_system_javascript.dart';
+
+import 'backend.dart';
+import 'namer.dart';
 
 typedef jsAst.Expression _ConstantReferenceGenerator(ConstantValue constant);