A few spec compliance changes to pass the w3c tests

- relatedTarget must be null if pointerevent is capturing
- `new PointerEvent(..) instanceof PointerEvent === true`
- Manual tests: https://github.com/w3c/web-platform-tests

src/PointerEvent.js

@@ -117,6 +117,9 @@
       );
     }
 
+    // make the event pass instanceof checks
+    e.__proto__ = PointerEvent.prototype;
+
     // define the buttons property according to DOM Level 3 spec
     if (!HAS_BUTTONS) {
       // IE 10 has buttons on MouseEvent.prototype as a getter w/o any setting
@@ -148,6 +151,9 @@
     return e;
   }
 
+  // PointerEvent extends MouseEvent
+  PointerEvent.prototype = Object.create(MouseEvent.prototype);
+
   // attach to window
   if (!scope.PointerEvent) {
     scope.PointerEvent = PointerEvent;

src/dispatcher.js

@@ -215,6 +215,10 @@
      * @return {Event} A PointerEvent of type `inType`
      */
     makeEvent: function(inType, inEvent) {
+      // relatedTarget must be null if pointer is captured
+      if (this.captureInfo) {
+        inEvent.relatedTarget = null;
+      }
       var e = new PointerEvent(inType, inEvent);
       this.targets.set(e, this.targets.get(inEvent) || inEvent.target);
       return e;