Add dart-ajax

Split out core-ajax and core-xhr from the previous PR.

I addressed some of Siggi's comments from the other PR. The others had to do with the state of the code that I ported, style and specific APIs. I tried to make as few changes as possible, so I didn't want to do those now to make tracking easier.

The demo works, but still requires that extra template for some reason.
We can't do:

var ajax = querySelector('dart-ajax') as DartCoreAjax;
ajax.onCoreResponse...

?