fix js_interop_test for browsers without Object.observe

the test assumed Dart timing, which would be next microtask, but since this is a JS element it is updated later.

Review URL: https://codereview.chromium.org//372193003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@38054 260f80e4-7a28-3924-810f-c04153c831b5

pkg/polymer/test/js_interop_test.dart

@@ -95,13 +95,22 @@ main() => initPolymer().run(() {
       // Text will update asynchronously
       expect(jsElem.shadowRoot.text, 'FOOBAR:40');
 
-      return new Future(() {
+      return _onTextChanged(jsElem.shadowRoot).then((_) {
         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);