Parser does not allow `!` before `.` (NNBD)

[stereotype441]

With --enable-experiment=non-nullable, the following code has a parse error:

class A {
  int? b = 1;
}
main() {
  var a = A();
  print(a!.b); // ERROR: Expected to find ')'
}

Note: I discovered this while trying to confirm whether we support ! in cascade expressions, e.g. a..foo()!.bar=3; (see discussion at dart-lang/language#293 (comment)). So we should double check this case too when fixing the bug.

[stereotype441]

@danrubel

[eernstg]

Also note that this CL contains a Dart.g grammar file that builds on one of Lasse's proposals to include support for ! as a selector.

This means that there is a complete grammar for the NNBD syntax (so any conflicts with the rest of the language would come up), and also that it's possible to experiment with it (tools/spec_parse.py). That grammar is not currently the source of truth for NNBD syntax, but it tracks dart-lang/language#293 closely.

[danrubel]

The current implementation treats ?. as a single token meaning that

  var foo = bar?.name;  // <--- valid - no space between ? and .
  var foo = bar? .name; // <--- invalid - space between ? and .

Should we do the same for !. so that it's consistent?

[bwilkerson]

No. The '!' operator is a postfix operator, and doesn't need to be followed by a period:

int x = y!;

The expression x!.y is equivalent to x! . y or even (x!).y.