Adding support for sound null safety

[guyluz11]

Moving to null safety should be top priority.

Without support for null safety this package will not get new users as everybody is moving to it.

It is stopping me from using this package and I am sure it is not just me.

[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

https://github.com/google/protobuf.dart/issues/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.

[terrabythia]

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