Reverts: flutter/engine#56738
Initiated by: jonahwilliams
Reason for reverting: speculative revert for framework failures.
Original PR Author: jonahwilliams
Reviewed By: {jason-simmons}
This change reverts the following previous change:
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.
This also updates the vsync waiter to use RunNowOrPostTask, so that we start vsync events as early as possible.
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