os: Null device

R=rsc, cw
CC=golang-dev
https://golang.org/cl/1854042
diff --git a/src/pkg/os/file_windows.go b/src/pkg/os/file_windows.go
index 4f7acbb..2c1c20c 100644
--- a/src/pkg/os/file_windows.go
+++ b/src/pkg/os/file_windows.go
@@ -17,6 +17,8 @@
 	usefirststat bool
 }
 
+const DevNull = "NUL"
+
 func (file *File) isdir() bool { return file != nil && file.dirinfo != nil }
 
 func openFile(name string, flag int, perm int) (file *File, err Error) {