2288 [path_provider] Add missing DartDocs (cla: yes)

2289 [url_launcher] Update deps and docs in url_launcher_web (cla: yes)

2292 Update CONTRIBUTING.md to allow mockito (cla: yes)

2293 [image_picker]fix a crash when a non-image file is picked. (cla: yes)

2296 [shared_preferences] Add missing DartDoc (cla: yes)

2297 [share] README update (cla: yes)