Re: [webcomponents] defined promise (#427)

`document.whenDefined("x-mytag")` is not very descriptive, I would suggest `document.whenElementDefined("x-mytag")` or something similar to `document.fonts.check("MyFont")` which also returns a promise, like `document.checkElement("x-mytag")`.

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/427#issuecomment-194657089