os: sleep 5ms after process has exited on windows

Fixes #2866.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5719047

Author: alexbrainman

src/pkg/os/exec_windows.go

@@ -33,6 +33,11 @@ func (p *Process) wait() (ps *ProcessState, err error) {
 		return nil, NewSyscallError("GetProcessTimes", e)
 	}
 	p.done = true
+	// NOTE(brainman): It seems that sometimes process is not dead
+	// when WaitForSingleObject returns. But we do not know any
+	// other way to wait for it. Sleeping for a while seems to do
+	// the trick sometimes. So we will sleep and smell the roses.
+	defer time.Sleep(5 * time.Millisecond)
 	defer p.Release()
 	return &ProcessState{p.Pid, syscall.WaitStatus{ExitCode: ec}, &u}, nil
 }