Make use of new dart api for compute #613

Job	Run time
Check formatting (dio)	15s
Check formatting (example)	10s
Check formatting (example_flutter_app)	14s
Check formatting (plugins/cookie_manager)	15s
Check formatting (plugins/http2_adapter)	14s
Check formatting (plugins/native_dio_adapter)	15s
Analyze Flutter libraries (example_flutter_app)	41s
Analyze Flutter libraries (plugins/native_dio_adapter)	50s
Analyze Dart-only libraries (dio)	23s
Analyze Dart-only libraries (example)	24s
Analyze Dart-only libraries (plugins/cookie_manager)	15s
Analyze Dart-only libraries (plugins/http2_adapter)	22s
Run unit tests with plugins	0s
Run unit tests with dio	0s
Run unit tests with Flutter plugins	0s
	4m 18s

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