[whatwg/dom] Event.returnValue legacy property (#222)

[`Event.returnValue`](https://developer.mozilla.org/en-US/docs/Web/API/Event/returnValue) seems to work in all modern browsers (Chrome, Safari, Edge, IE11) except Firefox, but it's not spec'd anywhere.
Testcase: http://jsbin.com/xisujo/edit?html,js,output

Consider specing it?

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