fetch the full HTML source in the debugger

Don't merge this yet! This needs a tiny patch in our tools to export the `fetch` function used here.

Opening the PR in case someone wants to go ahead and look at the code.
Yeah, it was a quick hack. I can take a look at this tomorrow afternoon.

Should be easy to get this in, the patch this requires landed on fx-team and (I believe) will be patch on gum.