MessageEvent#ports should contain transferred ports

[targos]

The following test currently fails:

'use strict';

const assert = require('assert');

const channel = new MessageChannel();
channel.port2.onmessage = function(event) {
  assert.strictEqual(event.ports.length, 1);
};

const channel2 = new MessageChannel();

channel.port1.postMessage('', [channel2.port1]);

Reference: https://html.spec.whatwg.org/multipage/comms.html#dom-messageevent-ports

[jasnell]

/cc @addaleax