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