go.tools/cmd/cover: preserve comments, as other build tools may need "// +build" directives.
go.tools/cmd/cover: preserve comments, as other build tools may need "// +build" directives.

Also: fix a crash when a file already contains 'import "sync/atomic"'.

R=r, gri
CC=golang-dev
https://codereview.appspot.com/14441052

d0f/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,8limbs/gotools,d0f/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,8limbs/gotools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,skyportsystems/go.tools,8limbs/gotools,d0f/go-zh.tools,webfd/go-zh.tools