Map does not work as expected in IE for empty string keys in the latest version

[fengzhenqiong]

Problem

The Map.prototype.get and Map.prototype.set does not work as expected in IE 11.

Details

I'm referring polyfill.js with script tag like this

<script src="//cdn.polyfill.io/v3/polyfill.min.js"></script>

And this will always refers to the latest published version of polyfill.js.

When I tried to use the Map in IE like this:

var map = new Map();
map.set("", "test value");
console.log(map.get(""));

the result will be undefined but not the expected value "test value";

Actually IE 11 does support Map.prototype.get and Map.prototype.set and they work fine, but the polyfill make them erroneous.

Problematic versions

I also tried the old versions of polyfill.js and it worked fine until version 3.42.0 and this problem starts to show up since version 3.43.0 and still exists in the newest version 3.52.3.

Additional information

Win 7 Enterprise x64 SP1
IE x64 11.0.9600.17843

[JakeChampion]

Looks like a regression from the performance refactor we did on Map and Set. This PR will fix it and ensure we don't regress again -- #557