WeakMap.prototype.get/has/delete may throw in old Chrome

[loganfsmyth]

Just encountered on a user's machine, so I didn't get to play around with it much, but they were getting TypeError: Invalid value used as weak map key which lead me to https://code.google.com/p/chromium/issues/detail?id=460083.

The user was still on Chrome 40, and our code was basically doing .get(null) and expecting a falsy value, which the spec defines as undefined 23.3.3.3 step #5. Instead, Chrome 40 was throwing an exception.