onBlack property Null safety

Chalk onBlack

chalkdart favors 'onXXXXX' style of specifying background colors because it makes chained methods list read better as a sentence (it's the Dart way). We include original Chalk 'bgXXXX' method names for users that prefer that scheme and for legacy compatability with original JS Chalk.

Implementation

Chalk get onBlack =>
    _createStyler(_ansiSGRModiferOpen(40), _ansiBgClose, this);