Add 'part of' in dart2js code.

lib/compiler/implementation/js_backend/constant_system_javascript.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;
+
 const JAVA_SCRIPT_CONSTANT_SYSTEM = const JavaScriptConstantSystem();
 
 class JavaScriptBitNotOperation extends BitNotOperation {
   const JavaScriptBitNotOperation();
 
   Constant fold(Constant constant) {
     if (JAVA_SCRIPT_CONSTANT_SYSTEM.isInt(constant)) {
       // In JavaScript we don't check for -0 and treat it as if it was zero.
       if (constant.isMinusZero()) constant = DART_CONSTANT_SYSTEM.createInt(0);
       IntConstant intConstant = constant;
@@ skipping 214 matching lines @@
     // At runtime, an integer is both an integer and a double: the
     // integer type check is Math.floor, which will return true only
     // for real integers, and our double type check is 'typeof number'
     // which will return true for both integers and doubles.
     if (s.element == compiler.intClass && t.element == compiler.doubleClass) {
       return true;
     }
     return compiler.types.isSubtype(s, t);
   }
 }