Breaks the build

««« original CL description
cmd/go: build test files containing non-runnable examples

Even if we can't run them, we should at least check that they compile.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/107320046
»»»

TBR=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/110140044