Add support for micro-tasks in Dartium.

tools/dom/src/native_DOMImplementation.dart

Index: tools/dom/src/native_DOMImplementation.dart
diff --git a/tools/dom/src/native_DOMImplementation.dart b/tools/dom/src/native_DOMImplementation.dart
index 2b760a209a8c4ad6216292c059628ad0f63078b3..7b123486440d280197e60513d6faaf667e81d774 100644
--- a/tools/dom/src/native_DOMImplementation.dart
+++ b/tools/dom/src/native_DOMImplementation.dart
@@ -531,6 +531,46 @@ get _pureIsolateTimerFactoryClosure =>
   throw new UnimplementedError("Timers on background isolates "
                                "are not supported in the browser"));
 
+class _ScheduleImmediateHelper {
+  MutationObserver _observer;
+  final DivElement _div = new DivElement();
+  Function _callback;
+
+  _ScheduleImmediateHelper() {
+    // Mutation events get fired as soon as the current event stack is unwound
+    // so we just make a dummy event and listen for that.
+    _observer = new MutationObserver(_handleMutation);
+    _observer.observe(_div, attributes: true);
+  }
+
+  void _schedule(callback) {
+    if (_callback != null) {
+      throw new StateError(
+          'Only one immediate callback can be scheduled at once');
+    }
+    _callback = callback;
+    // Toggle it to trigger the mutation event.
+    _div.hidden = !_div.hidden;
+  }
+
+  _handleMutation(List<MutationRecord> mutations, MutationObserver observer) {
+    var tmp = _callback;
+    _callback = null;
+    tmp();
+  }
+}
+
+final _ScheduleImmediateHelper _scheduleImmediateHelper =
+    new _ScheduleImmediateHelper();
+
+get _scheduleImmediateClosure => (void callback()) {
+  _scheduleImmediateHelper._schedule(callback);
+};
+
+get _pureIsolateScheduleImmediateClosure => ((void callback()) =>
+  throw new UnimplementedError("scheduleMicrotask in background isolates "
+                               "are not supported in the browser"));
+
 void _initializeCustomElement(Element e) {
   _Utils.initializeCustomElement(e);
 }