function extractEvents(topLevelType, targetInst, nativeEvent, nativeEventTarget) {
  var events = null;
  for (var i = 0; i < plugins.length; i++) {
    // Not every plugin in the ordering may be loaded at runtime.
    var possiblePlugin = plugins[i];
    if (possiblePlugin) {
      var extractedEvents = possiblePlugin.extractEvents(topLevelType, targetInst, nativeEvent, nativeEventTarget);
      if (extractedEvents) {
        events = accumulateInto(events, extractedEvents);
      }
    }
  }
  return events;
}
Allows registered plugins an opportunity to extract events from top-level
native browser events.

@return {*} An accumulation of synthetic events.
@internal