Fix dart2js tests, unparser initializer with closure.

lib/compiler/implementation/tree/unparser.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.
 
 // Returns null if no need to rename a node.
 typedef String Renamer(Node node);
 
 class Unparser implements Visitor {
-  final Renamer rename;
+  Renamer rename;
   StringBuffer sb;
 
-  Unparser() : this.withRenamer((Node) => null);
+  Unparser() {
+    rename = (Node node) => null;
+  }
   Unparser.withRenamer(this.rename);
 
   String unparse(Node node) {
     sb = new StringBuffer();
     visit(node);
     return sb.toString();
   }
 
   void add(SourceString string) {
     string.printOn(sb);
@@ skipping 481 matching lines @@
       sb.add(' ');
     }
     visit(node.name);
     if (node.typeParameters !== null) {
       visit(node.typeParameters);
     }
     visit(node.formals);
     add(node.endToken.value);
   }
 }