Add 'part of' in dart2js code.

lib/compiler/implementation/constant_system_dart.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 dart2js;
+
 const DART_CONSTANT_SYSTEM = const DartConstantSystem();
 
 class BitNotOperation implements UnaryOperation {
   final SourceString name = const SourceString('~');
   bool isUserDefinable() => true;
   const BitNotOperation();
   Constant fold(Constant constant) {
     if (constant.isInt()) {
       IntConstant intConstant = constant;
       return DART_CONSTANT_SYSTEM.createInt(~intConstant.value);
@@ skipping 347 matching lines @@
   bool isInt(Constant constant) => constant.isInt();
   bool isDouble(Constant constant) => constant.isDouble();
   bool isString(Constant constant) => constant.isString();
   bool isBool(Constant constant) => constant.isBool();
   bool isNull(Constant constant) => constant.isNull();
 
   bool isSubtype(Compiler compiler, DartType s, DartType t) {
     return compiler.types.isSubtype(s, t);
   }
 }