This is the supertype of all value types that can be passed to JavaScript code. Record types are the canonical counter-example: they are not subtypes of any because their values cannot be given to JavaScript.
This type supports a very limited set of Scala operations, the ones common to all values. Basically only reference equality tests and instance tests. It also supports all JavaScript operations, since all Scala objects are also genuine JavaScript values.
The type java.lang.Object in the back-end maps to AnyType because it can hold JS values (not only instances of Scala.js classes).
