Add 'part of' in dart2js code.

lib/compiler/implementation/scanner/string_scanner.dart

 // Copyright (c) 2011, 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 scanner;
+
 /**
  * Scanner that reads from a String and creates tokens that points to
  * substrings.
  */
 class StringScanner extends ArrayBasedScanner<SourceString> {
   final String string;
 
   StringScanner(String this.string, {bool includeComments: false})
     : super(includeComments);
 
@@ skipping 51 matching lines @@
     assert(0 <= terminal);
     assert(initial + terminal <= internalString.length);
     return new SubstringWrapper(internalString,
                                 begin + initial, end - terminal);
   }
 
   bool get isEmpty => begin == end;
 
   bool isPrivate() => !isEmpty && identical(internalString.charCodeAt(begin), $_);
 }