Element superclass is always HTMLElement

[galanovnick]

Description

No matter which parent your class have, iron-doc-element always displays HTMLElement.
If you try to replace superclass from the source JSON with another value, a computed reference in iron-doc-element will lead to https://baseurl/mixins/Superclass which is pretty weird. I'm not saying that it should find the real source URL of my superclass, but it's definitely shouldn't point to mixins.

class MyCustomElementExample extends MySuperclass { ... }

Expected outcome

The actual superclass is shown and its link does not refer to mixins.

Actual outcome

The HTMLElement superclass is shown.

[galanovnick]

The issue is probably connected to https://github.com/Polymer/polymer-analyzer/issues/750.