Ace and CodeMirror are the clear leaders in editors now.
http://ace.c9.io/
http://codemirror.net/
As Ace has massive backing behind it, and a long history that has clearly stabilised against an impressive and active Open Source project, this is what I think we should go with.
Finally, this one is implemented. It is on github in the feature__code_editing_update branch, which will be merged in for v11, which will be a relatively short development cycle. Thank you for the sponsorship Rishi, and sorry for the huge lead time. We're getting there though!
http://ace.c9.io/
http://codemirror.net/
As Ace has massive backing behind it, and a long history that has clearly stabilised against an impressive and active Open Source project, this is what I think we should go with.