fix nested bindings in polymer_expressions

pkg/polymer/test/nested_binding_test.dart

+// Copyright (c) 2013, 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 polymer.test.nested_binding_test;
+
+import 'dart:async';
+import 'dart:html';
+import 'dart:mirrors'; // import mirrors so we include String.length
+import 'package:polymer/polymer.dart';
+import 'package:unittest/unittest.dart';
+import 'package:unittest/html_config.dart';
+import 'package:unittest/matcher.dart';
+
+@CustomTag('my-test')
+class MyTest extends PolymerElement {
+  final List fruits = toObservable(['apples', 'oranges', 'pears']);
+
+  final _testDone = new Completer();
+
+  MyTest.created() : super.created();
+
+  _runTest(_) {
+    expect($['fruit'].text.trim(), 'Short name: [pears]');
+    _testDone.complete();
+  }
+
+  ready() {
+    onMutation($['fruit']).then(_runTest);
+  }
+
+}
+
+main() {
+  reflectClass(String); // use the mirrors import to suppress a warning.
+
+  initPolymer();
+  useHtmlConfiguration();
+
+  setUp(() => Polymer.onReady);
+
+  test('ready called', () => (query('my-test') as MyTest)._testDone.future);
+}