Add 'part of' in dart2js code.

lib/compiler/implementation/js/printer.dart

Index: lib/compiler/implementation/js/printer.dart
diff --git a/lib/compiler/implementation/js/printer.dart b/lib/compiler/implementation/js/printer.dart
index 17f25c4325efe43348bc1f04ab72685e942541f1..63dcbe9c9e951d74a1653790027c9c4e1838fe45 100644
--- a/lib/compiler/implementation/js/printer.dart
+++ b/lib/compiler/implementation/js/printer.dart
@@ -2,6 +2,8 @@
 // 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;
+
 class Printer implements NodeVisitor {
   final bool shouldCompressOutput;
   leg.Compiler compiler;
@@ -18,7 +20,7 @@ class Printer implements NodeVisitor {
         outBuffer = new leg.CodeBuffer(),
         danglingElseVisitor = new DanglingElseVisitor(compiler),
         namer = determineRenamer(compiler.enableMinification);
-  
+
   static Namer determineRenamer(bool shouldCompressOutput) {
     // TODO(erikcorry): Re-enable the MinifyRenamer after M1.
     return new IdentityNamer();