WeakMap keys require no primitive type constraint

[srenault]

The type checker shouldn't accept this:

var map = new WeakMap();
map.set('foo', 'bar');

A key must be of type "Object".

[danquirk]

The construct signature for WeakMap at the moment is:

new <K, V>(): WeakMap<K, V>;

which is why this is allowed. We don't currently have a way to specify that K must be an object type but not a primitive, but such a feature has been suggested in #1809.

[mhegazy]

We currently do not have a way to type that. if and when #1809 is available, we should revisit this bug.

[cpcallen]

Looks like #1809 is available. Should this be revisited? (Or if done in the mean time, have the Revisit tag removed?)