code: public void addHandler (@Nonnull final EJSEvent eJSEvent, @Nonnull final IHasJSCode aNewHandler)
    {
      ValueEnforcer.notNull (eJSEvent, "JSEvent");
      ValueEnforcer.notNull (aNewHandler, "NewHandler");
  
      CollectingJSCodeProvider aCode = m_aEvents.get (eJSEvent);
      if (aCode == null)
      {
        aCode = new CollectingJSCodeProvider ();
        m_aEvents.put (eJSEvent, aCode);
      }
      aCode.appendFlattened (aNewHandler);
    }
docs: Add an additional handler for the given JS event. If an existing handler is
  present, the new handler is appended at the end.
  
  @param eJSEvent
  The JS event. May not be <code>null</code>.
  @param aNewHandler
  The new handler to be added. May not be <code>null</code>.