Background: We are planning to make all Events constructable. For example,
CustomEvent already has the spec for its constructor
(http://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html#interface-customevent).

We propose a spec for MessageEvent constructor as follows:

[Constructor(DOMString type, optional MessageEventInit eventInitDict)]
dictionary MessageEventInit : EventInit {
    any data;
    DOMString origin;
    DOMString lastEventId;
    WindowProxy source;
    MessagePort[] ports;
}