primary-button.js

$( "#dialog" ).dialog({
	buttons: [
		{
			text: "a",
			"class": "primary",
			click: function() {
				alert( "a" );
			},
		},
		{
			text: "b",
			click: function() {
				alert( "b" );
			}
		}
	]
});

ui-dialog-button.js

$.widget( "ui.dialog", $.ui.dialog, {
	_createButtons: function() {
		this._super();
		this.uiButtonSet.children().addClass( "button" );
	}
});