Add 'part of' in dart2js code.

lib/compiler/implementation/tree/tree.dart

Index: lib/compiler/implementation/tree/tree.dart
diff --git a/lib/compiler/implementation/tree/tree.dart b/lib/compiler/implementation/tree/tree.dart
index 2e4c5afc0c6b8c61145c81d26a81481e67b8414c..83639181c83b6b5aaaca763d3aebc7083276b227 100644
--- a/lib/compiler/implementation/tree/tree.dart
+++ b/lib/compiler/implementation/tree/tree.dart
@@ -2,16 +2,16 @@
 // 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.
 
-#library('tree');
+library tree;
 
-#import('dart:math');
+import 'dart:math';
 
-#import('../scanner/scannerlib.dart');
-#import('../util/util.dart');
-#import('../util/characters.dart');
+import '../scanner/scannerlib.dart';
+import '../util/util.dart';
+import '../util/characters.dart';
 
-#source('dartstring.dart');
-#source('nodes.dart');
-#source('prettyprint.dart');
-#source('unparser.dart');
-#source('visitors.dart');
+part 'dartstring.dart';
+part 'nodes.dart';
+part 'prettyprint.dart';
+part 'unparser.dart';
+part 'visitors.dart';