fix js_interop_test for browsers without Object.observe

pkg/polymer/test/js_interop_test.dart

 // Copyright (c) 2014, 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.web.js_interop_test;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:js';
 import 'package:polymer/polymer.dart';
@@ skipping 77 matching lines @@
 
       // Because Polymer.js two-way bindings are just a getter/setter pair
       // pointing at the original, we will see the new value immediately.
       expect(dartElem.twoWay, 42);
 
       expect(interop['foobar'], 42);
 
       // Text will update asynchronously
       expect(jsElem.shadowRoot.text, 'FOOBAR:40');
 
-      return new Future(() {
+      return _onTextChanged(jsElem.shadowRoot).then((_) {

[Siggi Cherem (dart-lang), 2014/07/08 16:50:38]

could we replace this with:
  jsElem.onMutation(jsElem.shadowRoot).then
?

[Jennifer Messerly, 2014/07/08 18:39:18]

no, for two reasons:

* jsElem is a JS element so it won't expose onMutation; we'd need .callMethod:
https://github.com/dart-lang/core-elements/issues/45

* onMutation doesn't look for characterData changes, see "characterData: true"
below vs
https://github.com/Polymer/polymer-dev/blob/27b6f83923125d658806a2086543c91e0...

         expect(jsElem.shadowRoot.text, 'FOOBAR:42');
       });
     });     
   });
 });
 
+Future<List<MutationRecord>> _onTextChanged(Node node) {
+  var completer = new Completer();
+  new MutationObserver((mutations, observer) {
+    observer.disconnect();
+    completer.complete(mutations);
+  })..observe(node, characterData: true, subtree: true);
+  return completer.future;
+}
+
 testInterop(jsElem) {
   expect(jsElem.shadowRoot.text, 'FOOBAR');
   var interop = new JsObject.fromBrowserObject(jsElem);
   expect(interop['baz'], 42, reason: 'can read JS custom element properties');
 
   jsElem.attributes['baz'] = '123';
   return flush().then((_) {
     expect(interop['baz'], 123, reason: 'attribute reflected to property');
     expect(jsElem.shadowRoot.text, 'FOOBAR', reason: 'text unchanged');
 
@@ skipping 16 matching lines @@
 /// Calls Platform.flush() to flush Polymer.js pending operations, e.g.
 /// dirty checking for data-bindings.
 Future flush() {
   var Platform = context['Platform'];
   Platform.callMethod('flush');
 
   var completer = new Completer();
   Platform.callMethod('endOfMicrotask', [() => completer.complete()]);
   return completer.future;
 }