Fix restart/reload benchmark synchronization (#11282)

Changes introduced in
https://github.com/flutter/engine/commit/8ba522eeae35d8c70ada3c7b8e200ca2274f4f95
has removed from the `_flutter.listViews` the thread synchronization
side effect used during benchmarks.

The thread synchronization is restored via the new
`_flutter.flushUIThreadTasks` RPC.

Fixes https://github.com/flutter/flutter/issues/11241

Related https://github.com/flutter/engine/pull/3898

packages/flutter_tools/lib/src/vmservice.dart

diff --git a/packages/flutter_tools/lib/src/vmservice.dart b/packages/flutter_tools/lib/src/vmservice.dart
index 8aea4b3..9b255ce 100644
--- a/packages/flutter_tools/lib/src/vmservice.dart
+++ b/packages/flutter_tools/lib/src/vmservice.dart
@@ -1230,6 +1230,10 @@
 
   bool get hasIsolate => _uiIsolate != null;
 
+  Future<Null> flushUIThreadTasks() async {
+    await owner.vm.invokeRpcRaw('_flutter.flushUIThreadTasks');
+  }
+
   @override
   String toString() => id;
 }