This makes me think - is there any reason you couldn't compile Emacs or Vim in Emscripten, and adapt it for the web? Maybe with some kind of in-browser terminal emulator? Quick googling suggests no one has tried this yet... I find that surprising.
(The Vim bindings in CodeMirror[1] are surprisingly decent, but it's still not the real thing.)
Stuff like this is where search in its current form fails.
Action.IO allows for browser based coding, and potentially development in the cloud. Heroku started out doing development in the cloud before they pivoted to PAAS.
(The Vim bindings in CodeMirror[1] are surprisingly decent, but it's still not the real thing.)
[1] http://codemirror.net/demo/vim.html