It is based on Visual Studio Online. Its text editor component is non-trivial and Microsoft spent a lot of money on it. The editor (codename "Monaco") is from Erich Gamma (ex-IBM/Rational), the "Design Patterns" book author from Switzerland. Microsoft bought the Monaco editor with him and its the single reason why TypeScript exists and why JavaScript ES6 looks like it looks ("class" syntax) and why he leads the Visual Studio team: http://en.wikipedia.org/wiki/Erich_Gamma
The Monaco editor (coded in TypeScript) will never be open source, its their competitive advantage. The same reason why HTML5's contentEditable API will never be fixed in IE/Chrome/Safari as a web office suite is their competitive advantage of Microsoft Office365 Web Word , Google Docs and Apple iCloud Pages. With a bug-free contentEditable API would render also Monaco editor and many other web based rich text editors obsolete: https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement...