chai 4.3.5+ doesn't work on IE10 because WeakMap

[felix9]

chai 4.3.5 uses loupe, which has an unconditional use of new WeakMap(), which doesn't exist on IE10.

I realize that IE10 support is probably not something anyone cares about today, so I don't expect this to get fixed :)

chai 4.3.4 still works on IE10, for anyone else who runs into this problem.