Create mixin and create getter bugs

[FMorschel]

Repro:

extension on int {
  int get foo => bar;
}

For the above code, bar should trigger Create getter. And it should not trigger Create mixin.

I'll take a look at this.

[FMorschel]

Here is the CL for this https://dart-review.googlesource.com/c/sdk/+/424560