Use ASTVisitor as in new AST.

compiler/javatests/com/google/dart/compiler/parser/ParserRoundTripTest.java

 // 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.
 
 package com.google.dart.compiler.parser;
 
 import com.google.dart.compiler.CompilerTestCase;
 import com.google.dart.compiler.ast.DartToSourceVisitor;
 import com.google.dart.compiler.ast.DartUnit;
-import com.google.dart.compiler.util.DefaultTextOutput;
 
 /**
  * Tests, for each of the parser test examples, that {@link DartToSourceVisitor} produces
  * something parseable. It's an imperfect test, as it doesn't account for semantic changes,
  * but it catches a lot of common problems.
  */
 public class ParserRoundTripTest extends CompilerTestCase {
 
   public void testClasses() {
     roundTrip("ClassesInterfaces.dart");
@@ skipping 41 matching lines @@
 
   public void testStrings() {
     roundTrip("Strings.dart");
   }
 
   /**
    * Ensures that a unit can be parsed, re-serialized, and re-parsed without error.
    */
   private void roundTrip(String path) {
     DartUnit unit = parseUnit(path);
-
-    DefaultTextOutput out = new DefaultTextOutput(false);
-    new DartToSourceVisitor(out, false).accept(unit);
-    String src = out.toString();
+    String src = unit.toSource();
     parseUnit(path, src);
   }
 }