Fix any nullability and refactor typing model

[srujzs]

Major changes:

1. Fixes an issue where we assume that all JSAnys should be nullable, whereas all anys should be nullable instead.
2. Changes typing model to go from IDLType to "raw" type, which is either an IDL declaration type or a dart:js_interop type, to a Dart type reference. This makes it easier to add generic types later as well.

Minor changes:

1. Reuses _Type and renames it to _RawType instead of using a record.
2. Removes unnecessary "isReturn" parameters as undefined can only appear in a return type per the web IDL spec.
3. JSVoid -> void as it's more readable.

[srujzs]

Quite a large diff in the output but it's entirely either the void change or the any nullability change.

[sigmundch]

Looks great, only very minor optional suggestions below

[sigmundch]

style nit: consider removing the else here. Mostly it helps make more visible the conditions that are excluded from the rest of the method.

if (type1 == 'JSUndefined' || type2 == 'JSUndefined') {
  return ...;
}

if (type1 == type2) {
  return ...
}

If by chance we can make them fit as a single line, it would make it visibly more obvious too.

It may be possible if we split the undefined cases and add a few more locals:

final type1 = rawType1.type;
final type2 = rawType2.type;
final nullable1 = rawType1.type;
final nullable2 = rawType2.type;

if (type1 == 'JSUndefined') return _RawType(type2, true);
if (type2 == 'JSUndefined') return _RawType(type1, true);
if (type1 == type2) return _RawType(type1, nullable1 || nullable2);

// ...

[srujzs]

Thanks, this is more readable. I still don't love how the logic places the equality case in-between the two sentinel cases, but alas.

[sigmundch]

mmm... good point. If we can guarantee that every time we create a _Ratype with 'JSUndefined' we also use true for the nullability, then we could move the == first. WDYT?

[srujzs]

I like it, prevents a non-nullable JSUndefined from leaking into a _RawType. #89

[sigmundch]

style nit: this may be a nice use of the new pattern syntax :)

dartType = switch(dartType) {
  'JSBoolean' => 'bool',
  'JSString' => 'String',
  'JSInteger' => 'int',
  'JSDouble' => 'num',
  'JSNumber' => 'num',
  'JSUndefined' => 'void',
  _ => dartType,
};
if (dartType == 'void') nullable = false;

[srujzs]

Done, thanks!