Add support for flushing microtasks in dart_engine.h

Also add a sample for calling Dart functions which return/accept futures.

Change-Id: I932d0577709f4e068ccd3212797751ecb347c4f9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/408281
Commit-Queue: Ivan Inozemtsev <iinozemtsev@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>

runtime/engine/include/dart_engine.h

@@ -109,6 +109,17 @@ typedef void (*DartEngine_HandleMessageErrorCallback)(
 DART_EXPORT void DartEngine_SetHandleMessageErrorCallback(
     DartEngine_HandleMessageErrorCallback handle_message_error_callback);
 
+/**
+ * Drains the microtasks queue. Requires to be an active isolate.
+ *
+ * Normally the microtasks queue is drained after handling each
+ * isolate message, but when the engine calls into Dart, it might be
+ * required to manually drain the microtasks queue.
+ *
+ * \return Dart_Handle invocation result.
+ */
+DART_EXPORT Dart_Handle DartEngine_DrainMicrotasksQueue();
+
 /**
  * Handles a single message for an isolate.
  */