如何在Go GAE应用程序上可视化代码覆盖率信息?

I am developing a server with the latest Go GAE SDK. I am running tests after every change:

goapp test -test.v=true

I am using -cover to record coverage as described by goapp help testflag:

goapp test -cover -test.v=true -test.coverprofile=c.out
[..]
coverage: 53.8% of statements
ok      _/var/lib/jenkins/jobs/loyalty/workspace    30.464s

This completes successfully and prints the percentage of lines covered by tests. However, attempting to visualize the results fails:

goapp tool cover -html=c.out
cover: can't find "app.go": cannot find package "_/home/ingo/git/loyalty/" in any of:
/home/ingo/Downloads/go_appengine_sdk_linux_amd64-1.9.10/go_appengine/goroot/src/pkg/_/home/ingo/git/loyalty (from $GOROOT)
/home/ingo/git/loyalty/src/_/home/ingo/git/loyalty (from $GOPATH)

Does Go's cover tool only work on non-GAE apps? Do I have to package my app differently in order to visualize the coverage results?

I unsuccessfully asked this on golang-nuts before.

There is an open issue related to it. As a temporary workaround, I am running sed in between collecting and visualizing the coverage results.

goapp test -cover -test.v=true -test.coverprofile=c.out
sed -i -e "s#.*/\(.*\.go\)#\./\\1#" c.out
goapp tool cover -html c.out -o coverage.html