Use Null as return type for HelpCommand.run (#164)
Use Null as return type for HelpCommand.run (#164)

This may make it easier to understand choices around nullability by
making it explicit that the help command can never return a useful value
from it's `run` method.

Remove the unnecessary return value and return statement since a `Null`
return type may be treated similarly to `void`.
dart-lang/args