Update polymer children using polymer api

[jimfb]

For web components using polymer, we sometimes screw up the updates of children, because polymer uses a shady dom. This PR will use the polymer DOM api to update children IFF the user is using polymer, thereby ensuring that all DOM child operations happen correctly.

[jimfb]

@spicyj @sebmarkbage

[sophiebits]

Polymer is supposed to be completely transparent. If you're loading Polymer before React and React doesn't work, it's a Polymer bug.

[sophiebits]

ex: #1263 (comment)

[jimfb]

@spicyj: shady dom, not shadow dom.

Reference: https://www.polymer-project.org/1.0/articles/shadydom.html#shady-dom-is-born

[sebmarkbage]

This feels very shady indeed.

1. The hook is in the wrong place since it rechecks in the hot path for everyone.
2. It checks a global but not a module system based hook which is unfortunate since you can never use it with a module system.
3. It effectively claims the Polymer name since browsers can no longer add it to the global namespace without breaking the web. Usually it works because pages that rely on Polymer would shadow such a name. Maybe that's too late any donno.
4. It adds a special hook to a particular library, unfairly favoring this particular implementation rather than a different web components polyfill. If there was a more generic and standardized hook that was more generic, this wouldn't feel as bad.
5. Polymer isn't stable and popular enough to warrant this kind of special casing. jQuery, Immutable-js, Lodash lazy collections etc. would probably be more interesting to special case if we did that for anything.

[gaearon]

Closing per #5948 (comment). We can open a separate issue if this is something we want to track.