[analyzer] Incorrect extensions reported in a conflict

[eernstg]

Consider the following program:

extension E on int? { void foo() {} }
extension E1 on int { void foo() {} }
extension E2 on int { void foo() {} }
void main() => 1.foo();

This program causes the analyzer (9ff5ac1) to emit the following error message:

ERROR|COMPILE_TIME_ERROR|AMBIGUOUS_EXTENSION_MEMBER_ACCESS|/usr/local/google/home/eernst/lang/dart/scratch/202105/n001.dart|4|18|3|A member named 'foo' is defined in extensions 'E' and 'E1' and neither is more specific.

However, the conflict actually arises for E1 and E2, not for E and E1, so the error message is wrong. It seems likely that the problem lies in the error message and not in the underlying checks, because we get a correct error message if E is commented out, and we also (correctly) do not get any error messages if E2 is commented out.

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/198240