Make sure to preserve a space between `?` and `.`

[munificent]

We're working on two new language features:

• Null-aware elements
• Static access shorthands

These two features can be combined:

var list = <double>[? .infinity];

It's sort of like we're adding two new prefix operators (though the . isn't really a prefix operator). However, there is already a ?. operator and the lexical grammar tokenizes that as a single unit.

This means that if the formatter encounters a shorthand .foo expression inside a null-aware element ? expression, it must preserve a space between the ? and .. We have a similar edge case in the formatter around preserving a space between chained unary - operators like - -a so that it doesn't accidentally turn them into a prefix decrement.

[FMorschel]

@bwilkerson, I found this and looking at this code:

class MyClass {
  static MyClass? maybeClass;
  static MyClass definetlyClass = MyClass();

  List<MyClass> get list => [
    .definetlyClass,
    ?.maybeClass        // missing_identifier
    ? .maybeClass       // OK
  ];
}

Is there any way we could make a better warning here? If you think so, I can open an issue on the SDK for it, or mention this at dart-lang/sdk#59836.

[munificent]

Is there any way we could make a better warning here?

Good point. Even if the formatter does the right thing, this is still a footgun. Filed a language issue to discuss: dart-lang/language#4477