Failing at compile time requires that for each conversion
between two interface types the compiler compare the sets of
unexported methods to see if they come from different
packages.  Since this test will fail approximately never on
real code, and since it can't catch all cases of the problem,
I don't think it's worth testing in the compiler.  This CL
changes this test to look for a run-time panic rather than a
compile-time error.

R=gri, rsc1, iant2, rsc
CC=golang-dev
https://golang.org/cl/4332041