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 CloseEvent constructor as follows:

[Constructor(DOMString type, optional CloseEventInit eventInitDict)]
dictionary CloseEventInit : EventInit {
    boolean wasClean;
    unsigned short code;
    DOMString reason;
}