Add 'part of' in dart2js code.

lib/compiler/implementation/typechecker.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;
+
 class TypeCheckerTask extends CompilerTask {
   TypeCheckerTask(Compiler compiler) : super(compiler);
   String get name => "Type checker";
 
   static const bool LOG_FAILURES = false;
 
   void check(Node tree, TreeElements elements) {
     measure(() {
       Visitor visitor =
           new TypeCheckerVisitor(compiler, elements, compiler.types);
@@ skipping 972 matching lines @@
   }
 
   DartType visitStringNode(StringNode node) {
     compiler.unimplemented('visitNode', node: node);
   }
 
   DartType visitLibraryDependency(LibraryDependency node) {
     compiler.unimplemented('visitNode', node: node);
   }
 }