.getItem() failing since tslib >= 1.12.0 upgrade

[OwenPattison]

An issue has cropped up in my project whereby trying to use the getItem method blows up at this point: https://github.com/localForage/localForage/blob/master/dist/localforage.js#L920

This seems to be related to the tslib 1.12.0 and is also an issue in tslib 2.0.0

here's the error I get:

my current workaround is to downgrade to tslib 1.11.2