Run dart migrate

[lejard-h]

Run dart migrate everywhere without accepting changes. So we have // @dart=2.9 header on every file and all pubspec.yaml opt in to null safety

Also use observable package from github rep (see #34)

Fix #41

[lejard-h]

@GZGavinZhao Can we have a null-safety branch to merge this and keep master branch without null safety.

Or do you want to keep dev branch (I am not sure if it's building right now)

[GZGavinZhao]

Yes, null-safety sounds good to me. I think for now it's better to leave dev clean as I'm not sure what @dukefirehawk is up to.

[GZGavinZhao]

@lejard-h Done. You can create a PR to that branch now. I will fix the workflow error on master and merge the change to null-safety later.