Rework Unparser/unparse StringBuffer management.

tests/compiler/dart2js/unparser2_test.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.
 
 #import("../../../lib/compiler/implementation/scanner/scannerlib.dart");
 #import("../../../lib/compiler/implementation/elements/elements.dart"); // only need CompilationUnitElement
 #import("../../../lib/compiler/implementation/tree/tree.dart");
 #import("../../../lib/compiler/implementation/leg.dart"); // only need DiagnosticListener & Script
 
 // This is the number of spaces to trim from the test code.
@@ skipping 92 matching lines @@
   Script script = new Script(null, null);
   LibraryElement lib = new LibraryElement(script);
   CompilationUnitElement element = new CompilationUnitElement(script, lib);
   StringScanner scanner = new StringScanner(source);
   Token beginToken = scanner.tokenize();
   NodeListener listener = new NodeListener(diagnosticListener, element);
   Parser parser = new Parser(listener);
   parser.parseUnit(beginToken);
   Node node = listener.popNode();
   Expect.isTrue(listener.nodes.isEmpty());
-  return new Unparser().unparse(node);
+  return unparse(node);
 }
 
 class MessageCollector implements DiagnosticListener {
   List<String> messages;
   MessageCollector() {
     messages = [];
   }
   void cancel([String reason, node, token, instruction, element]) {
     messages.add(reason);
     throw reason;
   }
   void log(message) {
     messages.add(message);
   }
   void internalErrorOnElement(Element element, String message) {
     throw message;
   }
   void internalError(String message,
                      [Node node, Token token, Dynamic instruction,
                       Element element]) {
     throw message;
   }
 }