Adding support for sound null safety about dart_chromecast

[terrabythia]

@guyluz11 hm... interesting. protoc_plugin is probably only needed to generate the files in lib/proto. We can remove it!

[terrabythia]

@guyluz11 maybe I'm missing something, but null safety support was added 9 months ago in version 0.2.5 (see the changelog: https://github.com/terrabythia/dart_chromecast/blob/master/CHANGELOG.md).

[terrabythia]

I see what you mean now. It's complaining that one of the transitive dependencies doesn't support null safety. Unfortunately this is because the current protoc_plugin uses an old version of that library and there is no newer version yet.

If you can find a way to fix this anyway feel free to create a PR!

[guyluz11]

I have opened an issue for google after opening this issue

google/protobuf.dart#540 and this is related too.

As @JishnuM wrote for json_serializable maybe we can use dependency override here and it should work too.
In another project of mine I am generating the protoc files with protoc program and not the dart package protoc_plugin, so this is another option.

And I think protoc_plugin belongs in dev_dependencies and not in dependencies in case we go with keeping it.

Tell me what option you prefer that I will work on.

I think the better option is to get rid of it as it does not look like google keeps maintaining it.

[guyluz11]

@terrabythia can you try removing protoc_plugin and test if everything is working correctly?, I think it is not needed at all.