Split js_backend into multiple libraries

pkg/compiler/lib/src/js_backend/constant_handler_javascript.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.
 
-part of js_backend;
+import '../compile_time_constants.dart';
+import '../compiler.dart' show Compiler;
+import '../constants/constant_system.dart';
+import '../constants/expressions.dart';
+import '../constants/values.dart';
+import '../elements/elements.dart';
+import '../elements/visitor.dart' show BaseElementVisitor;
+import '../resolution/tree_elements.dart' show TreeElements;
+import '../tree/tree.dart';
+import 'constant_system_javascript.dart';
 
 /// [ConstantCompilerTask] for compilation of constants for the JavaScript
 /// backend.
 ///
 /// Since this task needs to distinguish between frontend and backend constants
 /// the actual compilation of the constants is forwarded to a
 /// [DartConstantCompiler] for the frontend interpretation of the constants and
 /// to a [JavaScriptConstantCompiler] for the backend interpretation.
 class JavaScriptConstantTask extends ConstantCompilerTask {
   DartConstantCompiler dartConstantCompiler;
@@ skipping 253 matching lines @@
     // TODO(ahe): This doesn't belong here. Rename this class and generalize.
     var closureClassMap = constants
         .compiler.closureToClassMapper.closureMappingCache
         .remove(node);
     if (closureClassMap != null) {
       closureClassMap
           .removeMyselfFrom(constants.compiler.enqueuer.codegen.universe);
     }
   }
 }