This is the Uber issue to track the flag flip in the Dart VM to only support sound null safety and deprecate legacy null safety mode.
List all the dependent issue that this change is blocked on here

• Delete execution of Dart VM benchmarks in legacy null safety mode #50092
• Need a mechanism to add a VM argument on the reply from dartdev isolate that would get set on the invocation #50090)
• Some Analyzer tests appear to fail in strong mode (Library 'package:test/my_macro.dart.macro' was compiled without sound null safety) #50100
• The --no-sound-null-safety flag doesn't appear to be functional when 'dart compile jit-snapshot' or 'dart compile kernel' is used #50079
• compile [exe|aot-snapshot] with --no-sound-null-safety does not work when the Dart VM is switched to sound null safety by default #50101
• 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 #50102
• Found 34 files excluded from sound null safety in webdev/ webdev#1640
• Tests in the co19_2 directory that run process do not use Platform.executableArguments when spawning processes co19#1509