Add 'part of' in dart2js code.

lib/compiler/implementation/tree/dartstring.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 tree;
+
 /**
  * The [DartString] type represents a Dart string value as a sequence of Unicode
  * Scalar Values.
  * After parsing, any valid [LiteralString] will contain a [DartString]
  * representing its content after removing quotes and resolving escapes in
  * its source.
  */
 abstract class DartString implements Iterable<int> {
   factory DartString.empty() => const LiteralDartString("");
   // This is a convenience constructor. If you need a const literal DartString,
@@ skipping 185 matching lines @@
       for (int i = 0; i < 3; i++) {
         code = source.next();
         value = value * 16 + hexDigitValue(code);
       }
       return value;
     }
     return code;
   }
 }