Changes the following shell callbacks to flush the dart event loop:
* OnPlatformViewSetViewportMetrics
* OnPlatformViewDispatchPointerDataPacket
* OnPlatformViewDispatchPlatformMessage
* OnPlatformViewSetSemanticsEnabled
* OnPlatformViewSetAccessibilityFeatures
Using a new TaskRunner API RunNowAndFlushMessages. If the task runner can run tasks on the current thread, this will immediately invoke a callback and then post an empty task to the event loop to ensure dart listeners fire.
Unlike https://github.com/flutter/engine/pull/56738, does not touch the vsync API - which looks like it depends on scheduling behavior today, at least for iOS.
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