Switch bulk Object.defineProperties to seperate Object.defineProperty…

… calls, to make IE happy; with Object.defineProperties, event.target always returning null even directly after setting it to a non-null value

lib/media/eme_manager.js

@@ -545,7 +545,7 @@ shaka.media.EmeManager.prototype.onSessionMessage_ = function(event) {
   shaka.log.info('onSessionMessage_', event);
   shaka.asserts.assert(this.drmScheme_);
   this.requestLicense_(
-      event.target || event.customTarget,
+      event.target,
       /** @type {!shaka.player.DrmSchemeInfo} */ (this.drmScheme_),
       event.message);
 };

lib/util/fake_event_target.js

@@ -107,19 +107,28 @@ shaka.util.FakeEventTarget.prototype.removeEventListener =
  */
 shaka.util.FakeEventTarget.prototype.dispatchEvent = function(event) {
   // Overwrite the Event's properties.  Assignment doesn't work in most
-  // browsers.  Object.defineProperties seems to work, although some browsers
+  // browsers.  Object.defineProperty seems to work, although some browsers
   // need the original properties deleted first.
   delete event.srcElement;
   delete event.target;
   delete event.currentTarget;
-  Object.defineProperties(event, {
-    'srcElement': { 'value': null, 'writable': true },
-    // target may be set many times if an event is re-dispatched.
-    'target': { 'value': this, 'writable': true },
-    // Dupe of target prop, as browsers like IE do not allow this to be redefined, even if CustomEvent is deleted from window.
-    'customTarget': { 'value': this, 'writable': true },
-    // currentTarget will be set many times by recursiveDispatch_().
-    'currentTarget': { 'value': null, 'writable': true }
+
+  var srcElement_ = null;
+  Object.defineProperty(event, "srcElement", {
+    get: function () { return srcElement_; },
+    set: function (value) { srcElement_ = value; }
+  });
+
+  var target_ = this;
+  Object.defineProperty(event, "target", {
+    get: function () { return target_; },
+    set: function (value) { target_ = value; }
+  });
+
+  var currentTarget_ = null;
+  Object.defineProperty(event, "currentTarget", {
+    get: function () { return currentTarget_; },
+    set: function (value) { currentTarget_ = value; }
   });
 
   return this.recursiveDispatch_(event);