Rework Unparser/unparse StringBuffer management.

tests/compiler/dart2js/unparser_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('dart:uri');
 #import('parser_helper.dart');
 #import('mock_compiler.dart');
 #import("../../../lib/compiler/compiler.dart");
 #import("../../../lib/compiler/implementation/leg.dart", prefix:'leg');
 #import("../../../lib/compiler/implementation/dart_backend/dart_backend.dart");
 #import("../../../lib/compiler/implementation/elements/elements.dart");
 #import("../../../lib/compiler/implementation/tree/tree.dart");
 
 testUnparse(String statement) {
   Node node = parseStatement(statement);
-  Expect.equals(statement, node.unparse());
+  Expect.equals(statement, unparse(node));
 }
 
 testUnparseMember(String member) {
   Node node = parseMember(member);
-  Expect.equals(member, node.unparse());
+  Expect.equals(member, unparse(node));
 }
 
 const coreLib = @'''
 #library('corelib');
 class Object {}
 interface bool {}
 interface num {}
 interface int extends num {}
 interface double extends num {}
 interface String {}
@@ skipping 818 matching lines @@
   testDoubleMains();
   testInterfaceDefaultAnotherLib();
   testStaticAccessIoLib();
   testLocalFunctionPlaceholder();
   testMinification();
   testClosureLocalsMinified();
   testParametersMinified();
   testTypeVariablesInDifferentLibraries();
   testDeclarationTypePlaceholders();
 }