Update github's syntax highlighting for NNBD

[stereotype441]

(Not really an analyzer issue, but I'm filing it as an analyzer issue on @aadilmaan's recommendation because we don't really have another good category for it)

We need to update the Atom grammar for Dart (https://github.com/dart-atom/dart/tree/master/grammars) to support NNBD, so that Dart code that shows up in github (and in gists) is properly syntax highlighted.

[stereotype441]

@devoncarew FYI

[lrhn]

What syntax highlighting will we need for NNBD? As I read it, it's regexp-based substring matching for highlighting, not actual parsing, so there are limits to its precision.

There are two new built-in identifiers: required (perhaps only highlight when it occurs after a ( or ,) and late. Those should be recognized.

The remaining grammar changes are the type modifier (type?), expression non-null cast (expr!), and some more null-aware operations (?.., ?.[...], ?.(...)).

The current state is:

// @dart = 2.7
main({required int! z}) {
  late final x = [print("run me now")];
  int? x = 42;
  for (var y in [x!]) {
    print(x?.toString());
    print([x]?.[0])
    print([x]?..[0] = 1);
  }
}

The build-in identifiers obviously need to be recognized.
The ? after the type is already highlighted.
The ! and ? are colored red, but not combined with the following ., so maybe we can even improve that (should ?. have a different color than .?).

[srawlins]

Mind if we close this in favor of #41882?

[devoncarew]

Note that the system-of-record for the syntax highlighting for github has moved to https://github.com/dart-lang/dart-syntax-highlight/blob/master/grammars/dart.json.