Set tests that have mixin errors as 2.19.

Mark tests that contain errors about using a class as a mixin to use
language version 2.19 where that's not an error.

This may not fix all of the tests because it's the language version of
the library where the class is declared that matters, not where the
class is used as a mixin. But most tests have all of their declarations
in the same library, so this should fix most.

Change-Id: I910439ebd2f10f731418dc588b7e4619a0841c16
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/285923
Reviewed-by: Jake Macdonald <jakemac@google.com>
Commit-Queue: Jake Macdonald <jakemac@google.com>