Revert "[co19] Enable tests migrated to null-safety"
Revert "[co19] Enable tests migrated to null-safety"

This reverts commit 9a588de9f4dad07e9ffce1012eef1c1470817da3.

Reason for revert: The tests need additional fixes before they can be enabled, and this CL makes it harder to land the support for running
the tests on tryjobs.

Original change's description:
> [co19] Enable tests migrated to null-safety
> 
> Change-Id: I738f5be6fe60aa4addb925101eb7f2912627ec8b
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/148527
> Reviewed-by: Sergey Grekhov <sgrekhov@unipro.ru>
> Reviewed-by: William Hesse <whesse@google.com>
> Commit-Queue: William Hesse <whesse@google.com>

TBR=whesse@google.com,29642742b6693024c89de8232f2e2542cf7eedf7@google.com,sgrekhov@unipro.ru

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: If758a0041214693aa6cd90f76b3e1a4f4db78c5b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/149283
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk