`Create member` fixes don't handle generics

Repro:

```dart
class A<T> {}

T? f<T>(A<T> a) => a.method();
T? g<T>(A<T> a) => a.getter;
void h<T>(A<T> a, T? o) => a.setter = o;
T? Function() i<T>(A<T> a) => a.methodTearoff;
```

Whereas if you select the `extension member` fixes, they do. We should probably make them coherent here.

I'll take a look at this.