Avoid crash if WeakSet is not available

[jgoz]

WeakSet is not available in IE11, so using react-window in development mode causes a runtime error.

This simply falls back to Set if WeakSet is not available. This could have memory implications since Set will hold onto the references indefinitely, but that only affects the one browser and only in development mode. It's also a much smaller impact on bundle size vs., say, a polyfill.

[bvaughn]

I know this is a small change in terms of LOC, but I'm having a strong initial negative reaction to it. I think because I don't like the idea of leaking memory (even in DEV mode) when there's a reasonable workaround, e.g.:

if (process.env.NODE_ENV !== 'production') {
  if (typeof window.WeakMap === 'undefined') {
    window.WeakMap = require('es6-weak-map/polyfill');
  }
}

[jgoz]

Yeah, I definitely see your point. What about simply not initializing the sets if WeakSet isn't available? IE11 wouldn't get the warnings in dev mode, but at least it wouldn't crash.

[bvaughn]

I like this approach much better. Thanks