Not just editable, but there's also autocompletion of strings (not quite intellisense, as it's just one file at a time) and token/symbol recognition.
Plus, this editor looked REALLY familiar to me. I started looking.
I looked over at the Windows Azure Portal, where developers can write node.js to make web services directly in the browser. Here we've got dropbox autocomplete, tooltips with syntax errors and even some basic symbolic refactoring!
Below you can see the editor in Azure Mobile Services throwing a tooltip syntax error. Is this happening on the server?
Then I went to look at TFS Online's stuff at http://tfs.visualstudio.com/ where I made an account http://hanselman.visualstudio.com to host private Git repos for side projects.
I brought up F12 tools just to check.
That's pretty unambiguous. Looking at the CSS by just clicking on editor.main.css. The "vs" in the div's class point to a vs-theme.css that I presume is to set the colors and make the text editor look familiar.
Looking in editor.main.js, it's all minified, but it's cool to see.