Isn't Shumway more of a confirmation for my concern than a counter example? Shumway is running ActionScript which is extremely close to JavaScript semantically though somewhat more strict and allows for optional typing.

It agrees on numeric types, but it has very large differences like namespaces, classes, etc. Look at how complex the Shumway VM is.

AS3 also has things like pointers (via domain memory). The closest thing to that in current JS is asm.js.