Add factory-method for scheduleMicrotask, as well as a (failing) test.

sdk/lib/_internal/lib/async_patch.dart

Index: sdk/lib/_internal/lib/async_patch.dart
diff --git a/sdk/lib/_internal/lib/async_patch.dart b/sdk/lib/_internal/lib/async_patch.dart
index bb1aa7cbef020f73fc884a7ef7ddbd17913b4127..07b1cd20fc4b7776cd91603f62fb1ca311ca055f 100644
--- a/sdk/lib/_internal/lib/async_patch.dart
+++ b/sdk/lib/_internal/lib/async_patch.dart
@@ -22,7 +22,7 @@ patch Timer _createPeriodicTimer(Duration duration,
 }
 
 patch class _AsyncRun {
-  patch static void _enqueueImmediate(void callback()) {
+  patch static void _scheduleImmediate(void callback()) {
     // TODO(9002): don't use the Timer to enqueue the immediate callback.
     _createTimer(Duration.ZERO, callback);
   }