3964 [multiple_web] Adapt web PlatformView widgets to work slotted. (cla: yes, waiting for tree to go green, p: google_maps_flutter, p: url_launcher, p: video_player, platform-web)

3968 [video_player] Migrate integration tests to NNBD (cla: yes, p: video_player)

3975 [video_player] Update README.md (cla: yes, waiting for tree to go green, p: video_player)

3979 [video_player] Add URL for exoplayer to maven for all dependents (cla: yes, p: camera, p: video_player, platform-android)