paulmillr/es6-shim — pull request #147 by ckknight (2013-10-03 20:14:21+00:00)
Title: Don't require descriptor support to implement Map and Set. If descriptors are not supported, then the 'size' property is overwritten directly rather than proxying to '_size'.

Technically this would mean that size is overwritable in non-ES5 implementations, but aside from that, it works as expected.