Flow analysis not accepting uninitialized final variables that are definitely assigned exactly once

[stereotype441]

From an IM conversation with @Hixie

The following code should be accepted with NNBD enabled (@leafpetersen confirmed):

final int foo;
switch (bar) {
  case 'a': foo = 1; break;
  case 'b': foo = 2; break;
  default: foo = 3;
}

It is currently rejected by the analyzer. We need to get this working and make sure there is a language_2 test case for it (so that we won't forget to ensure that it works on CFE as well).

[scheglov]

@leafpetersen I cannot find where this is specified. The closest statement that I was able to find requires that the variable is also late: "It is not a compile time error to write to a final variable if that variable is declared late and does not have an initializer.".

[leafpetersen]

@scheglov We're discussing some of the corners of this feature here, but will have spec to you shortly.

[leafpetersen]

The spec for this landed, and the implementation work is tracked here: #43100