[dom-parts] Browser updating parts list from getParts()

[tbondwilkinson]

@justinfagnani @mfreed7 @sorvell @kevinpschaaf and a few folks from Angular have been thinking about the DOM parts API fresh.

Taken with #990 and #992, the browser needs to return a list of parts that is accurate with what is represented in the DOM. The browser also needs to keep that list updated when nodes are added or removed from the DOM or when new parts are constructed. Ideally this list should be in DOM-order as well, to make walking the parts as easy as possible.

How the browser should keep this list live may be easiest with a cached list of parts that is invalidated. For a full discussion of how the browser might do this book-keeping, see this discussion on a new proposal based on DOM parts.

[keithamus]

WCCG had their spring F2F in which this was discussed. Present members of WCCG identified an action item to take the topic of DOM Parts and break it out into extended discussions. You can read the full notes of the discussion (#978 (comment)) in which this was discussed, heading entitled "DOM Parts API".

As this issue pertains to DOM parts, I'd like to call out that #999 has been raised for extended discussions and this topic may be discussed during those sessions.