outerHTML isn't always serializing is= when it should

[pshaughn]

This is WPT custom-elements/parser/serializing-html-fragments.html; 2 out of 3 cases fail.
In the first case, an element is created from a custom element constructor and added to the DOM. Its serialization is missing the is= attribute.
In the second case, an element is created with a regular constructor and an additional is attribute, with the name in the is= string not being a registered custom element class. The serialization omits the ineffectual is= attribute and WPT expects to include it.
In the third case, an custom element has .setAttribute('is',...) called on it for a different string than its real registered class name; the serialization correctly reflects the change.
Safari fails the same two cases as Servo, apparently in the same way.

[pshaughn]

The second case is https://html.spec.whatwg.org/multipage/parsing.html#serialising-html-fragments 4.2.

The first case is this code:
https://github.com/web-platform-tests/wpt/blob/fda9fa30b9c18f3689f09384c0d4f104c6e6fd4d/custom-elements/parser/serializing-html-fragments.html#L12-L14
I haven't yet found the specification that explains when or how calling the registered constructor sets either the is content attribute or the is value.

[pshaughn]

https://html.spec.whatwg.org/multipage/dom.html#html-element-constructors 7.8 is where the is value gets set. It looks like this involves looking at the "newTarget" value from within a codegen-defined constructor, much like #25394 needs.