Auto-completion at factory constructors show instance members

If you have a code like:

```dart
class A {
  A();

  final bool isEmpty = false;

  factory A.a() {
    ^
    return A();
  }
}
```

At the `^` you ask for auto-completion, it shows `isEmpty` as an option but it is invalid since we are in a `factory` constructor. It should be auto-completing the same way as for a `static` function.

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FYI @DanTup @bwilkerson 