Jason Simmons 4ef69b5dbb Ensure that Picture::RasterizeToImage destroys Dart persistent values on the UI thread (flutter/engine#8182)
The DartPersistentValue used to hold the image callback is tied to a
Dart isolate.  Destructing the DartPersistentValue requires entering
the isolate and must be done on the UI thread.

Fixes https://github.com/flutter/flutter/issues/29379
2019-03-28 12:37:18 -07:00
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