auto convert fundamental JavaScript types and DOM objects

[ErikGrimes]

Polymer Elements tend to expose these either as properties or as the detail for custom events. IronAjax for example fires 'error' which is a map containing the IronRequest and the JavaScript Error. IronRequest contains a reference to the JavaScript XMLHttpRequest.

Fundamental Types

[jakemac53]

cc @jacob314 I am not sure how much of this could/should be done by dart:js, and how much should be done by Polymer. I am ok with initial implementations for this sort of thing in Polymer but it seems like for the fundamental types dart:js should eventually own the conversion.