[various] prepare for 19.0.0 dev.1 release #268

Job	Run time
Build Android example app (3.19)	3m 31s
Run Dart Analyzer	51s
Build iOS example app (stable channel)	2m 11s
Build iOS example app (3.19)	3m 20s
Build Android example app (stable channel)	3m 48s
Build macOS example app (3.19)	3m 16s
Build macOS example app (stable channel)	2m 49s
Dart formatting checks	25s
Build Linux example app (stable channel)	1m 2s
Build Linux example app (3.19)	1m 18s
Build Windows example app (3.19)	3m 4s
Build Windows example app (stable channel)	5m 38s
Run integration tests (Android)	4m 49s
Run integration tests (iOS)	9m 13s
Run all unit tests (Android)	5m 3s
Run all unit tests (Dart)	53s
Run all unit tests (Windows)	3m 6s
	54m 17s

Provide feedback