Add a test for operator[] unparse.

tests/compiler/dart2js/parser_helper.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.
 
 #library('parser_helper');
 
 #import("dart:uri");
 
 #import("../../../lib/compiler/implementation/elements/elements.dart");
 #import("../../../lib/compiler/implementation/tree/tree.dart");
@@ skipping 29 matching lines @@
 Node parseStatement(String text) =>
   parseBodyCode(text, (parser, tokens) => parser.parseStatement(tokens));
 
 Node parseFunction(String text, Compiler compiler) {
   Element element = parseUnit(text, compiler, compiler.mainApp).head;
   Expect.isNotNull(element);
   Expect.equals(ElementKind.FUNCTION, element.kind);
   return element.parseNode(compiler);
 }
 
+Node parseMember(String text) =>
+    parseBodyCode(text, (parser, tokens) => parser.parseMember(tokens));
+
 class MockFile {
   final filename = '<string>';
   final text;
   MockFile(this.text);
 }
 
 Link<Element> parseUnit(String text, Compiler compiler,
                         LibraryElement library) {
   Token tokens = scan(text);
   Uri uri = new Uri(scheme: "source");
   var script = new Script(uri, new MockFile(text));
   var unit = new CompilationUnitElement(script, library);
   int id = 0;
   ElementListener listener = new ElementListener(compiler, unit, () => id++);
   PartialParser parser = new PartialParser(listener);
   compiler.withCurrentElement(unit, () => parser.parseUnit(tokens));
   return unit.topLevelElements;
 }
 
 // TODO(ahe): We define this method to avoid having to import
 // the scanner in the tests. We should move SourceString to another
 // location instead.
 SourceString buildSourceString(String name) {
   return new SourceString(name);
 }