JSIL can run pretty much all modern .NET code with correct semantics in JS (i.e. the standard library + real enterprise apps/games), but the performance cost to match semantics is pretty significant. And then there's stuff you just can't do, like weak references :(
WeakMap isn't weak references. Easy mistake to make (most TC39 members don't understand the difference). WeakMap is a weird approximation of weak properties, where the weak relationship is in the opposite direction.
It was not the only one, it crops back up periodically and the purpose/nuances of weakrefs have to be re-explained each time. To be fair, there are a lot of subtleties involved...
