Did some more debugging on this one. The problem occurs in dartium/src/third_party/WebKit/LayouTests/dart/resources/pong.html :

return event.source.postMessage('echo: ${event.data}', ...)

which throws the error:

CONSOLE ERROR: Exception: type '_DOMWindowCrossFrame' is not a subtype of type 'EventTarget' of 'function result'.

basically event.source is listed as being only an EventTarget (https://api.dartlang.org/apidocs/channels/be/#dart-dom-html.MessageEvent@id_source)

and so this fails in checked mode because we were trying to remove the "implements EventTarget" from WindowBase, which _DOMWindowCrossFrame implements.

So in summary, MessageEvent.source can be an EventTarget, but ALSO a WindowCrossFrame. We need to change our return type to deal with this so we don't fail in checked mode!

Removed the owner.
Changed the title to: "Modify type of MessageEvent.source because its type could be not an EventTarget".

What are the docs for the source property?

The w3C docs say source should be Window: http://www.w3.org/TR/2008/WD-html5-20080610/comms.html

whatwg claims it can be a WindowProxy or MessagePort: http://www.whatwg.org/specs/web-apps/current-work/multipage/comms.html#messageevent