front end appears to run some weak null safety unit tests which all fail when the VM is flipped to run by default in strong null safety mode

[a-siva]

The front end appears to run some weak null safety tests, I presume this will continue to be needed to support dart2js. These tests appear to fail when the VM is switched to run by default in strong null safety mode. Some adjustments may need to be made to the runner to use the --no-strong-null-safety mode or these tests need to be skipped.

Please see https://ci.chromium.org/p/dart/builders/try/front-end-linux-release-x64-try/67459?