Use ol.isDef when checking booleans

src/ol/interaction/dragrotateinteraction.js

@@ -1,5 +1,6 @@
 goog.provide('ol.interaction.DragRotate');
 
+goog.require('ol');
 goog.require('ol.ViewHint');
 goog.require('ol.events.ConditionType');
 goog.require('ol.events.condition');
@@ -68,7 +69,7 @@ ol.interaction.DragRotate.handleDragEvent_ = function(mapBrowserEvent) {
   var offset = mapBrowserEvent.pixel;
   var theta =
       Math.atan2(size[1] / 2 - offset[1], offset[0] - size[0] / 2);
-  if (this.lastAngle_ !== undefined) {
+  if (ol.isDef(this.lastAngle_)) {
     var delta = theta - this.lastAngle_;
     var view = map.getView();
     var rotation = view.getRotation();