[flutter_tools] boolArg() should be non-nullable, and throw if the flag doesn't exist

https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/runner/flutter_command.dart#L1526

Discussion: https://github.com/flutter/flutter/pull/105242#discussion_r888395925