We leave the nsIDOMEventTarget* versions fallible for now, but this makes the
common case a lot simpler. Note that this means that pushing a null JSContext,
a bug, is no longer handled at runtime. But I think we should just assert
against it, since there are already callers that don't check the return value.
The goal here is to get rid of this crap entirely, and make nsCxPusher always
push. But that's a scary change, so we do it in chunks. This patch, in particular,
should have zero behavioral change. This means preserving some very wrong behavior.
For instance, currently SafeAutoJSContext never pushes a damn thing, because the
safe JSContext doesn't have an associated nsIScriptContext. We preserve this
behavior, and in fact convert various similarly-buggy consumers to
SafeAutoJSContext, so that we can hoist the behavioral change into a subsequent
patch.
