go/types: "user-defined object without package" error while typechecking crypto/tls #28282
Description
$ gotip version
go version devel +ff99a61e5e Fri Oct 19 14:11:52 2018 +0000 linux/amd64
The go/types test where it typechecks the whole standard library is currently failing on the crypto/tls package:
$ cd src/go/types
$ gotip test -run=Stdlib
--- FAIL: TestStdlib (9.24s)
stdlib_test.go:243: /home/alberto/go/src/crypto/tls/handshake_client_test.go:1651:76: user-defined object without package: func (interface).Error() string
stdlib_test.go:243: /home/alberto/go/src/crypto/tls/handshake_server_test.go:1439:76: user-defined object without package: func (interface).Error() string
FAIL
exit status 1
FAIL go/types 9.263s
Spotted on the longtest builder.
cc @griesemer