This is just the tool proper; stitching into "go test" will be a separate CL.
Tests are missing - they'll come once it's integrated - but it can handle,
perhaps correctly, all of src/pkg/...
The basic approach is to rewrite the source to add annotations that will
track coverage; the rewritten source must of course be compiled and
run after this tool has done its job.
R=adonovan
CC=golang-dev
https://golang.org/cl/10102043
