Add 'part of' in dart2js code.

lib/compiler/implementation/constant_system.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;
+
 interface Operation {
   final SourceString name;
   bool isUserDefinable();
 }
 
 interface UnaryOperation extends Operation {
   /** Returns [:null:] if it was unable to fold the operation. */
   Constant fold(Constant constant);
   apply(value);
 }
@@ skipping 46 matching lines @@
   bool isInt(Constant constant);
   /** Returns true if the [constant] is a double at runtime. */
   bool isDouble(Constant constant);
   /** Returns true if the [constant] is a string at runtime. */
   bool isString(Constant constant);
   /** Returns true if the [constant] is a boolean at runtime. */
   bool isBool(Constant constant);
   /** Returns true if the [constant] is null at runtime. */
   bool isNull(Constant constant);
 }