Failures on Migrate void tests to Null Safety.

[lrhn]

There are new test failures on Migrate void tests to Null Safety..

The tests

language/void/return_future_future_or_void_async_error1_test/none Pass (expected CompileTimeError)
language/void/return_future_or_future_or_void_sync_error2_test/none Pass (expected CompileTimeError)
language/void/return_future_or_void_sync_error4_test/none Pass (expected CompileTimeError)

are failing on configurations

cfe-strong-linux
cfe-weak-linux
dart2js-hostasserts-strong-linux-x64-d8
dart2js-hostasserts-weak-linux-x64-d8
dartdevk-strong-linux-release-chrome
dartdevk-weak-linux-release-chrome
dartk-strong-linux-debug-x64
dartk-strong-linux-release-x64
dartk-weak-asserts-linux-debug-x64
dartk-weak-asserts-linux-release-x64

The tests are generally checking that return; is not allowed unless the return type/future value type is void, dynamic or Null, and not, for example FutureOr<void>.