Work around lack of error event type info in demo

The demo listens for shaka.Player.ErrorEvent, but it doesn't have type
information for that type.  This is because the type is defined only
in the docs, and not in the externs.  There is no actual class defined
for the event types.

The extern generator should be improved to handle this, but in the
meantime, we will use square brackets.

This was caught by a compiler upgrade.

Issue shaka-project#2528

Change-Id: I55fc5fcc7f391f3155c1edc712064c5091fa363e

demo/main.js

@@ -1584,7 +1584,9 @@ shakaDemo.Main = class {
    * @private
    */
   onErrorEvent_(event) {
-    this.onError_(event.detail);
+    // TODO: generate externs automatically from @event types
+    // This event should be shaka.Player.ErrorEvent
+    this.onError_(event['detail']);
   }
 
   /**