Change the URL-space exposed by the browser server.

This is a step towards Dartium support. Rather than only serving the compiled JS
and the HTML necessary to run it, the entire package is now served with the
compiled JS and HTML wrappers overlayed on top. This will allow Dartium to load
all necessary sources from the server, and incidentally also allows tests to
access assets.

See #60
Closes #27

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1076803003