21060 Cherrypick Dart 5e8b31a5fa48 (cla: yes)

21061 Remove the type parameter from SkObjectFinalizationRegistry. (cla: yes, waiting for tree to go green)

21069 Add missing  header (cla: yes)

21075 Disable a flaky Scenarios test (cla: yes)

Merged PRs in flutter/plugins from 2020-07-09T05:19:00.000 to 2020-09-10T18:17:00.000

There were 65 pull requests.

970 [share] Add sharing file support (android & ios) (cla: yes, in review)

2824 [google_sign_in] Bridge google_sign_in and googleapis. (cla: yes)