Private superclasses lead to dead references

[Hixie]

Consider this setup:

class _Private {
  /// This is the documentation for "foo".
  final bool foo;
}

/// See the [foo] documentation.
class Public extends _Private { }

You'll get the error message (paraphrasing a little) "warning: no canonical library found for test._Private.foo, not linking from _Private.foo referred to by Public".

But Public.foo appears in the documentation with the text from _Private.foo, so it seems like Public should just link to its own foo when it refers to [foo].

Marking P1 as "Broken links, few or on edge cases".

[jcollins-g]

This is related to #1173, but probably fixable independent of that.

[jcollins-g]

Fixed in #1524.