Migrate to null safety

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/176486 will (hopefully) migrate package:analyzer to null safety.
But at least several tests are failing, because they expect that package:linter is available, and rules are registered. But we cannot make it available because importing an unmigrated library into a migrated test library file leads to a run error (to prevent accidentally think that you are null safe while you are not). So, we disable these tests.

Also most of the clients of package:analyzer externally (analyzer_cli) and internally (g3_analyzer_cli) register lint rules. This means that these clients cannot be made null safe until package:linter is migrated.

Then we will able to re-enable tests.

[pq]

This is exciting!

I started on this a bit ago and will restore my branch and get back to it.

Let's touch base and coordinate when everyone is back from holiday?

[pq]

Some dependencies to migrate:

• markdown: publish null-safe package version tools#1277
• grinder: publish null-safe package version google/grinder.dart#369
• http: Migration to null safety http#501