3630 [file_selector] Return a non-null value from getSavePath on web (cla: yes, p: file_selector, platform-web, waiting for tree to go green)

3633 Make executor an instance property (cla: yes, p: shared_preferences, platform-android, waiting for tree to go green)

3635 [image_picker_for_web] Bump version for NNBD stable (cla: yes, p: image_picker, platform-web, waiting for tree to go green)

3636 [in_app_purchase] migrate playing billing library to v3 (cla: yes, p: in_app_purchase, platform-android, waiting for tree to go green)

3639 [shared_preferences] Don’t create additional Handler when method channel is called. (cla: yes, p: shared_preferences, platform-android, waiting for tree to go green)