3996 [google_sign_in] Add iOS unit and UI tests (cla: yes, waiting for tree to go green, p: google_sign_in, platform-ios)

3998 [path_provider] Add iOS unit tests (cla: yes, waiting for tree to go green, p: path_provider, platform-ios)

4000 [url_launcher] Fix breaking change version conflict (cla: yes, waiting for tree to go green, p: url_launcher)

4004 Check in regenerated C++ example registrants (cla: yes, waiting for tree to go green, p: path_provider, p: url_launcher, p: shared_preferences, platform-windows, platform-linux)

4008 [in_app_purchase] Explanation for casting details to implementations (cla: yes, waiting for tree to go green, p: in_app_purchase)