Add if not null operator: !?

[mathieudevos]

Simple suggestion, allow the negation of the if null operator: ?? with the following: !?.

This prevents you from running functions that cannot handle null input.

Example:

double foobar(double input) {
  return input / 2;
}

void printIt(dynamic input) {
  print(input.toString());
}

void main() {
  final foo = null;

  // Suggested method
  printIt(foo !? foobar(foo));

  // Current method
  printIt((foo == null) ? null : foobar(foo));
}

The current style just has a weird way of writing with the null repetition.
Alternatively we could allow the unary prefix to apply: !foo ?? foobar(foo) would also be acceptable.

[natebosch]

You might want to add a comment on #361 which proposes something similar but uses !! instead of !?

[lrhn]

Agree, this is a duplicate of #361.