UI.TextArea: Disable spellcheck.

editor/js/libs/ui.js

@@ -353,6 +353,7 @@ UI.TextArea = function () {
 	dom.className = 'TextArea';
 	dom.style.padding = '2px';
 	dom.style.border = '1px solid #ccc';
+	dom.spellcheck = false;
 
 	dom.addEventListener( 'keydown', function ( event ) {