ValueOf() panics if x is not one of the expected types.

Change-Id: I1105e46bd09a5ab13c162b77c1c50cc45bce27a2
GitHub-Last-Rev: 34a88ce8206954d94f0a884ab7f6494116c54a2d
GitHub-Pull-Request: golang/go#28846
Reviewed-on: https://go-review.googlesource.com/c/150138Reviewed-by: Richard Musiol <neelance@gmail.com>