This makes us pass http://www.w3c-test.org/html/tests/approved/canvas/2d.pattern.image.null.html http://www.w3c-test.org/html/tests/approved/canvas/2d.pattern.image.undefined.html

Boris: not-ready-for-cedar as in post 2.2?

That was my thinking on the "it makes us throw where we don't throw now" premise. But I would be open to being convinced otherwise here.... (e.g. if my premise is wrong or if we have good reason to believe that no one depends on our old behavior).

We already throw NS_ERROR_NOT_AVAILABLE, this is just a change of which exception we throw.