False analyzer warnings when using nullable extensions

Consider the following example:
```dart
void main() {
final _A? e = _Aaa();
switch (e) {
case null: // using Null() works as expected
e.name; // should be able to call .name

case _Aa():
print(e.name);
}

// using is Null works as expected
if (e == null) {
e.name; // should be able to call .name
}
}

sealed class _A {}

sealed class _Aa extends _A {}

final class _Aaa extends _Aa {}

extension on _Aa? {
String get name {
return switch (this) {
null => 'null',
_Aaa() => 'a:a:a',
};
}
}
```
It seems when explicitly checking for `null` the analyzer still treats the variable as `T?`, making it impossible to use an extension method defined on a more specific type and null.

A solution is to use an `is Null` check, however, in that case the `type_check_with_null` lint triggers and prompts the user to use a `== null` check instead.

Start with the Dart example in the issue and reproduce the analyzer warning for the `switch (e)` null case, then compare it with the `e == null` and `is Null` checks. Trace the analyzer's type promotion for nullable extensions; done means the explicit null case permits the extension and does not require the conflicting workaround lint.

Written by the indexing model from the issue text.