fix js_interop_test for browsers without Object.observe

pkg/polymer/test/js_interop_test.dart

Index: pkg/polymer/test/js_interop_test.dart
diff --git a/pkg/polymer/test/js_interop_test.dart b/pkg/polymer/test/js_interop_test.dart
index e4bc6aa530781cb1ea2e2997f272713378c012d4..154cd01d8a232b6882738281d1f6fee22d93af41 100644
--- a/pkg/polymer/test/js_interop_test.dart
+++ b/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((_) {

[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);