Fyi, the Brackets file IO layer is indeed drop-in replaceable: https://github.com/adobe/brackets/wiki/File-System-Implement.... The core team only maintains the standard desktop file IO backend, but others can be swapped in.

That's only one piece of making an editor run in-browser and cross-browser of course, but it's an important piece.

Also -- there are actually a couple ChromeOS ports of Brackets out there already (such as Tailor - https://chrome.google.com/webstore/detail/tailor/mfakmoghean...), but I'm nto sure if any of them are being actively maintained.