Linter: public_member_api_docs doesn't seem to require docs for mixins

[goderbauer]

Describe the issue
The public_member_api_docs lint doesn't seem to warn when a public mixin (or any methods defined in it) do not have a doc comment.

To Reproduce
Enable the lint, the following sample doesn't trigger it:

mixin Foo {
  void foo() {
    // nothing to do.
  }
}

Expected behavior
Since they are part of the public API, the lint should require doc comments on them.

[pq]

Agreed. This one is showing it's age (predating mixins 😬 ) -- I'll update. Thanks!

[bwilkerson]

We should validate that it works for extensions as well.

[pq]

😊