Location is going to gain some members through an implements statement, like
Location implements Blah;
but we need those imported members from Blah to be [Unforgeable] in addition to all the existing ones on Location. So let's allow [Unforgeable] directly on interface Location, to indicate that all of its members (including the ones from the implements statement) to be unforgeable.
toString/valueOf on Location also need to be unforgeable.
See also this thread: http://lists.w3.org/Archives/Public/public-script-coord/2012JulSep/0144.html