co19/Language/Variables/final_

[scheglov]

With this null safety spec change, it is not an error to declared a final local variable without an initialized.
It is an error to read one unless it is definitely assigned.

The change to analyzer will approve following tests:

co19/Language/Variables/final_t04
co19/Language/Variables/final_t05
co19/Language/Variables/final_t06
co19/Language/Variables/final_t07

[leafpetersen]

cc @eernstg @athomas

[eernstg]

These four tests are all covered by the new approach to final locals: They can be declared with no initializer, and it is then allowed to assign to them when they are definitely unassigned (so there's no predecessor to the current location in the flow graph where the variable is assigned), and it is allowed to read them when they are definitely assigned (so every path in the flow graph up to the current location contains an assignment to that variable).