Split js_backend into multiple libraries

pkg/compiler/lib/src/js_backend/constant_emitter.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.
 
-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);
 
 typedef jsAst.Expression _ConstantListGenerator(jsAst.Expression array);
 
 /**
  * Generates the JavaScript expressions for constants.
  *
  * It uses a given [constantReferenceGenerator] to reference nested constants
  * (if there are some). It is hence up to that function to decide which
@@ skipping 306 matching lines @@
           [value, argumentList]);
     }
     return value;
   }
 
   @override
   jsAst.Expression visitDeferred(DeferredConstantValue constant, [_]) {
     return constantReferenceGenerator(constant.referenced);
   }
 }