Analyzer reports invalid matches for valid type checks on extension types

[jakemac53]

See the following example, using a private extension type to model a union type:

void main() {
  foo(1.asIntOrString);
  foo('1'.asIntOrString);
}

void foo(_IntOrString intOrString) {
  switch(intOrString) {
    case int():
      print('int!');
    case String():
      print('String!');
    default:
      throw StateError('Something else! ${intOrString.runtimeType}');
  }
}

extension type _IntOrString(Object _) {}

extension StringAsIntOrString on String {
  _IntOrString get asIntOrString => _IntOrString(this);
}

extension IntAsIntOrString on int {
  _IntOrString get asIntOrString => _IntOrString(this);
}

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/330981