fix: Fix thread-safety issues when accessing dartMessageHandlerManager_ #164

Job	Run time
Build macOS (2.10.5)	3m 45s
Build Android on Ubuntu (2.10.5)	4m 16s
Build Android on Windows (2.10.5)	9m 8s
Build macOS (3.x)	3m 59s
Build iOS (2.10.5)	3m 45s
Build Android on Windows (3.x)	8m 11s
Build Android on Ubuntu (3.x)	3m 59s
Build iOS (3.x)	3m 53s
Build Web on Ubuntu (2.10.5)	1m 55s
pub.dev scores check	1m 39s
Build Windows (2.10.5)	7m 41s
Flutter code style check	1m 8s
Build Windows (3.x)	4m 35s
android integration test	17m 5s
Build Web on Ubuntu (3.x)	1m 31s
ios integration test	7m 46s
windows integration test	4m 44s
ios integration test with use_frameworks	7m 21s
macos integration test	4m 3s
pub publish check	1m 6s
Run flutter unit test (2.10.5)	1m 33s
Run flutter unit test web (2.10.5)	1m 33s
Run flutter unit test (3.x)	1m 25s
Run flutter unit test web (3.x)	1m 39s
	1h 47m 40s

Provide feedback