Fixes 'cannot call method then on undefined' error when using HMR with a config
whose target is "webworker".
Probably should have happened with 2245c4acca
The way that the hot loader is included in a bundle breaks in Java 8's
Nashorn engine, because of a missing semicolon. This change introduces
a placeholder value that can be correctly substituted when building
a bundle, but is otherwise inert.
Note that I don't particularly like this change, so if there's a better
fix, I'd like to hear about it.