go/build: build constraint fails to exclude go:binary-only-package directive

[joshlf]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   1.7
2. What operating system and processor architecture are you using (go env)?
   darwin/amd64
3. What did you do?
   Build this package using go build -tags build_source with no binary version present.
4. What did you expect to see?
   The package should have built successfully since the // +build !build_source tag in binary.go should have excluded the //go:binary-only-package directive in that same file.
5. What did you see instead?
   missing or invalid package binary for binary-only package ...

[gopherbot]

CL https://golang.org/cl/31577 mentions this issue.