[Analyzer] Implement changes to null safety checking for local variables.

[leafpetersen]

Small set of changes to the errors and warnings around local variables landed here.

Tests for the the definite assignment based errors landed here.

Tests for the inference and promotion section to follow.

cc @scheglov

[scheglov]

First portion - reading.
https://dart-review.googlesource.com/c/sdk/+/159382

co19 should be updated.
dart-lang/co19#913

[scheglov]

We have one test that fails: tests/language/nnbd/definite_assignment/definite_assignment_error_test.dart
AFAIK this is #42990

[leafpetersen]

New tests for the inference and initialization portion landed here

Failing: language/nnbd/inference/variable_initialization_promotion_test
Passing: language/nnbd/inference/variables_initialized_to_null_test

[stereotype441]

All of these tests are now passing.