Add quick fix for making local variable non-final

[greglittlefield-wf]

This currently exists for final fields (related: #25316), and would be nice to have for local variables as well.

This would make it a little less painful to have the prefer_final_locals lint turned on.

Current behavior in Dart 2.0.0-dev.59.0:

[pq]

@greglittlefield-wf: sorry for the slow reply on this.

@scheglov, @bwilkerson: I'm assuming there's no good reason not to do this one? Seems reasonable and straight-forward?

[bwilkerson]

I see no reason not to support this, and yes, it should be fairly easy to extend the current quick fix to handle this case as well.

[scheglov]

I don't see any reason not to implement this, and it should be easy.

[greglittlefield-wf]

Awesome! Would you accept a patch for this?

[bwilkerson]

Yes, we're always happy to take contributions.

[greglittlefield-wf]

❤️