[video_player] Improve DartDocs and test coverage (flutter#2286)

Also adds a lint to prevent undocumented APIs going forward.

[shared_preferences] Add missing DartDoc (flutter#2296)

Also adds a lint to prevent further undocumented methods.

[share] README update (flutter#2297)

Adds a little more info on how to use the plugin. Also adds linter
options to prevent any undocumented APIs from being added in the future.

[path_provider] Add v2 embedding support for (flutter#2284)

[image_picker]fix a crash when a non-image file is picked. (flutter#2293

)

Migrate plugins to use e2e tests. (flutter#2282)

Migrate plugins to use e2e tests. (flutter#2282)

[connectivity] Fix reachability stream for iOS (flutter#2281)

[webview_flutter] Add async NavigationDelegates (flutter#2257)

Previously all navigation decisions needed to be made synchronously in
Dart. The platform layers are already waiting for the MethodChannel
callback, so just changed the return type to be
`FutureOr<NavigationDecision>'. This is a change to the method signature
of `WebViewPlatformCallbacksHandler#onNavigationRequest`, but that
interface appears to only be meant to be implemented internally to the
plugin.

[url_launcher] DartDoc and test improvements (flutter#2274)

- Add a lint to url_launcher and url_launcher_platform_interface to
prevent undocumented public members.
- Add some unit tests.