tipgodoc breaks tool installation #9381
Description
When installing the tools (using go get golang.org/x/tools/cmd/... as recommended by http://golang.org/doc/install/source), the build fails because tipgodoc does not define a main function in package main.
$ go get -u golang.org/x/tools/cmd/...
# golang.org/x/tools/cmd/tipgodoc
runtime.main_main: main.main: not defined
runtime.main_main: undefined: main.main
(Insert plea for version pinning in go get here, or at least separate release and development branches for the tools repo)