Dispatch Flash events asynchronously (#3700)

Flash blocks until the javascript side of ExternalInterface callbacks complete and swallows any exceptions generated during that process. To avoid performance issues and missed exceptions, trigger events from Flash asynchronously.

src/js/tech/flash.js

@@ -485,8 +485,16 @@ Flash.checkReady = function(tech) {
 // Trigger events from the swf on the player
 Flash.onEvent = function(swfID, eventName) {
   const tech = Dom.getEl(swfID).tech;
-
-  tech.trigger(eventName, Array.prototype.slice.call(arguments, 2));
+  const args = Array.prototype.slice.call(arguments, 2);
+
+  // dispatch Flash events asynchronously for two reasons:
+  // - Flash swallows any exceptions generated by javascript it
+  //   invokes
+  // - Flash is suspended until the javascript returns which may cause
+  //   playback performance issues
+  tech.setTimeout(function() {
+    tech.trigger(eventName, args);
+  }, 1);
 };
 
 // Log errors from the swf