Crap. I've already banished Babel from my toolchain -- the Babel ES6 shim throws a warning in Firefox about modifying the prototype of an object, and from what I can tell that will kill or limit both V8 and SpiderMonkey optimizations from that point on.

Back to plan A: Don't use Lodash in client code unless I'm willing to use the whole thing (so that I can get the chaining optimizations, which cherry picking doesn't support anyway).