A function on a type that is put in an interface must not be assumed to
not need a context pointer when it isn't directly put in a function
pointer, because the interface call side won't know this and pass an
extra parameter with that extra function pointer.
This is usually not a problem but WebAssembly has strict checks on
function signatures so this resulted in an error when running
src/examples/test/test.go.
This is one step towards removing unnecessary special casts in most
cases. It is also part of removing as much magic as possible from the
compiler (the pragma is explicit, the special name is not).
