[webcomponents] HTML Imports

So, Anne just reopened this bug:

https://www.w3.org/Bugs/Public/show_bug.cgi?id=22305

To bring in the discussion here and provide some context, a bunch of us got
together at the Mozilla Summit in Brussels to discuss the current state of
web components so that we could figure out what platform could implement
(the how and when) in Firefox.

What is the global object that scripts in the imported resource are
executed against?

My expectation would be that once imported, scripts execute against the
global/window context of the document doing the importing (presumably the
'master document' of a developer's application).

And, if the script is executed against the global/window object of the main
document, can and should you be able to access the imported document?

- Angelina