4027 [in_app_purchase] Avoid nullable Future.value (cla: yes, p: in_app_purchase, platform-ios)

4028 Fix typo; s/unnecessary_imports/unnecessary_import (cla: yes, waiting for tree to go green)

4029 [video_player] Remove pre-stable warning from README (cla: yes, p: video_player)

4033 Link to this repo’s guide in the PR template (cla: yes)

4035 [in_app_purchase] Only register transactionObservers when someone is listening to purchaseUpdates (cla: yes, p: in_app_purchase, platform-ios)

4037 [flutter_plugin_tools] Migrate publish and version checks to NNBD (cla: yes)