[webcomponents] "Creating an element" needs to be more precise, for upgrades (#424)

In doing https://github.com/w3c/webcomponents/commit/2d07c73a1f5a00bf3f4655995273772fa57b1a02 I realized that we need to be more precise on exactly when and how upgrades are enqueued.

@annevk, I could use help figuring this out. Stuff like cloning, for example, needs to enqueue the callback. Its current language seems wrong:

> Let copy be a node that implements the same interfaces as node, and fulfills these additional requirements, switching on node: 

Are "the same interfaces" just HTMLElement?

What other APIs or parts of specs can create potentially-custom element nodes? So far I can think of:

- cloning
- createElement/createElementNS (already fixed)
- parser (known todo)

Is that really all on the platform?

One way to fix this would be to mint a new "create an element" operation which all of these must call.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/424