There is no shortcut for this. But it is easy to add with user script
var editors = require("ext/editors/editors");
require("ext/commands/commands").addCommand({
name: "focusEditor",
exec:function() {
if (editors.currentEditor)
editors.currentEditor.focus();
},
bindKey: "ctrl-alt-w"
});
if you can think of a good shortcut for this suggest it on https://github.com/ajaxorg/cloud9/issues/2032#issuecomment-9427700 too