This fixes an issue with port.on(event, listener) when event coincides with Object property, in which case an exception is thrown

For example, if one creates a Panel object and then tries to add a listener to 'watch' event on it like so:

panel.port.on("watch", function() {});

One'll get a 'listeners.indexOf is not a function' exception at runtime. This is because in this case a value of a nonstandard Object.prototype.watch property is returned and not event listeners array as excepted.