XPIDL has the requirement that [scriptable] interfaces with [notxpcom]
methods or attributes are [builtinclass]. Currently, if you don't
explicitly mark something builtinclass when it should be, then the
XPIDL compiler will just silently treat it like builtinclass. This
means that you can cause the JS implementation of an XPCOM to start
failing without any warning by marking a method notxpcom.

This patch instead makes it an error. A prior patch fixed the existing
instances in the tree that relied on the implicit behavior.

I also added a test that we reject such classes missing builtinclass
at compile time, as well as classes that inherit from builtinclass
interfaces without themselves being builtinclass. I left behind a part
of the runtime test for this behavior, but now this test just ensures
that you can't implement a [builtinclass] interface in JS.

Differential Revision: https://phabricator.services.mozilla.com/D30984

--HG--
extra : moz-landing-system : lando