I've done it using mixin of SimpleTextArea and ValidationTextArea:
define(["dojo/_base/declare", "dojo/_base/lang", "dijit/form/SimpleTextarea", "dijit/form/ValidationTextBox"],
function(declare, lang, SimpleTextarea, ValidationTextBox) {
return declare('dijit.form.ValidationTextArea', [SimpleTextarea, ValidationTextBox], {
constructor: function(params){
this.constraints = {};
this.baseClass += ' dijitValidationTextArea';
},
templateString: "<textarea ${!nameAttrSetting} data-dojo-attach-point='focusNode,containerNode,textbox' autocomplete='off'></textarea>"
})
})
See also my answer in Dojo validation of a textarea