* Stable null safety release camera_platform_interface

* Update minimum plugin_platform_interface version

* Update version of cross_file to 0.3.1

Migrates the app-facing package to null safety. Includes replacing Mockito with a custom fake/mock.

Fixes an issue where the example didn't handle dialogs being canceled, which was highlighted by the NNBD migration. (Previously, they would cause null assertions at runtime, which wasn't noticed during development. NNBD for the win!)

Fixes flutter/flutter#75235

`io.flutter.embedded_views_preview` is not required since version `1.0.0`, so doesn't need to be in the example.

The action won't work on the simulator works on physical iOS device which was not mentioned here so it was added

Co-authored-by: Michael Klimushyn <[email protected]>
Co-authored-by: Stuart Morgan <[email protected]>

…lutter#3642)

Migrates to NNBD.
Replaces Mockito-based fakes with test's Fake.

… leading dots. (flutter#3451)

…nel is called. (flutter#3639)

To avoid intra-repo plugin conflicts during the NNBD stable migration,
`plugin_platform_interface` allowed either 1.x or 2.0. However, 1.0.x
isn't null-safe so this can cause apps that don't have all their
packages fully updated can fail to run in strong mode (due to having an
old local `plugin_platform_interface`.

Now that everything has been updated, we can bump all the minimums so
that people updating their plugins will get new versions of the
dependency.

Stable NNBD release for the camera package.

With this PR I also migrated the example App to sound null safe.

Replaces the problematic Mockito mock with a manual mock that handles
null and non-null types correctly.

Removes the unit test of the example app, since it's not adding any
actual coverage.

Removes the opt-out now that the underlying issue is fixed

…utter#3664)