This has recently happened to a site of mine which needs to go to a client tomorrow. ??
Anyway, I read all the messages in this thread. Downloaded the Google Libraries – no fix. I then disabled all plugins – no fix. Changed theme to Twenty Eleven – no fix. I then checked other Javascript settings etc – no fix.
So, I hovered over Dashboard and chose ‘Updates’. I then chose ‘Reinstall now’. The version I am running is 3.4.2.
My editors now work. I also noticed there was a bug with featured images section too.
Hope this helps someone.