```swift
let c1 = JSClosure { _ in .undefined }
consume c1
```
did not release the `JSClosure` itself and it leaked the underlying
JavaScript closure too because `JSClosure` -> JS closure thunk ->
Closure registry entry -> `JSClosure` reference cycle was not broken
when using FinalizationRegistry. (Without FR, it was broken by manual
`release` call.)
Note that weakening the reference does not violates the contract that
function reference should be unique because holding a weak reference does
deinit but not deallocate the object, so ObjectIdentifier is not reused
until the weak reference in the registry is removed.

The test suite was not properly releasing the closures but they have not
been revealed as a problem because those closures were leaked
conservatively.

...tract
This additional information will help developers to find the root cause