Try supporting ES6 modules

Author: CarterLi

src/angular-gridster.js

@@ -14,9 +14,10 @@
 		factory(root.angular, window.ResizeObserver);
 	}
 }(this, function(angular, ResizeObserver) {
-
 	'use strict';
 
+	ResizeObserver = ResizeObserver.default || ResizeObserver;
+
 	var _utils = {
 		//region Test via a getter in the options object to see if the passive property is accessed
 		supportsPassive: (function() {