Refactoring operations should use var/final/const as appropriate

[stevemessick]

Original issue: flutter/flutter-intellij#6304

Summary: refactoring operations that create new declarations should use final and const where appropriate. Currently, var is used everywhere.

[bwilkerson]

It seems like the Dart analyzer ought to be able to determine that a declaration should be var if its redefined, final it is isn't, and const if it is a constant (not sure about that last).

I wish it were that simple. The problem, of course, is that the variable didn't exist before the extract operation, so it's guaranteed that it isn't re-assigned yet. What we can't know is whether the user will assign a new value after this operation.

The point about const is interesting. If the original expression is a const expression, then perhaps we should use const for it. (It isn't guaranteed that that's the right thing either, but it is an interesting signal.)

That said, the original point is valid and something we have been discussing. The question is, what signals can we use to determine the user's preferences? One signal that comes to mind is style-based lints that have been enabled. In this case, it seems clear that if the user has enabled prefer_final_locals then they probably want the variable to be declared using final. The question is whether that's sufficient.

[asashour]

Duplicate of #49561, but this has feedback.