Update dom/interfaces.html to current spec

This is based on top of pull request #358, and will almost certainly not work without that, because it uses IDL features only supported by webidl2.js.  I don't know if I should make such requests separate or just include that one in this.

This will cause the file to stop working until [testharness.js pull request #43](https://github.com/w3c/testharness.js/pull/43) is accepted.  So, don't accept this one before you accept that one.