Flutter with Dart 2.1 mixins does not display mixins separately

[jcollins-g]

cc @yjbanov

While testing flutter/flutter#22435 with 0.23.0, discovered that Mixins are still listed with Classes somehow in sidebars and in the library page.

[jcollins-g]

Double-checked and in at least some cases we do display mixins in the right section, so now I need to narrow down how the mixins in flutter/flutter#22435 differ from my test cases.

[jcollins-g]

Dartdoc was still using type checks on MixinElementImpl in some places to detect mixins, but since with heavy reexporting ClassElementHandles can be returned from _exportedNamespace (and Flutter makes use of reexporting a great deal), we were hitting assertion cases when they were used inside flutter.

Fix coming up.