3149 Add windows directory to examples (cla: yes)

3150 Configure analyzer for plugins that are migrated to nnbd (cla: yes)

3152 [share] Replace deprecated Environment.getExternalStorageDirectory() call on Android. (cla: yes)

3153 [google_sign_in] Fix merge mistake. (cla: yes)

3154 Prepare url_launcher for the Link widget (cla: yes, feature)

3155 [web] Implement Link for web (cla: yes, feature)

3156 [google_maps_flutter_web] Fix InfoWindow snippets. (cla: yes)

3157 Sync nnbd branch (cla: yes)

3159 [video_player_platform_interface] Migrate to null safety (cla: yes)