os: add Expand function to evaluate environment variables.

Fixes #1258.

R=gri, msolo, bradfitzpatrick, r2
CC=golang-dev
https://golang.org/cl/2988041
diff --git a/src/pkg/os/Makefile b/src/pkg/os/Makefile
index 0b0881c..e4b6fb9 100644
--- a/src/pkg/os/Makefile
+++ b/src/pkg/os/Makefile
@@ -8,6 +8,7 @@
 GOFILES=\
 	dir_$(GOOS).go\
 	error.go\
+	env.go\
 	exec.go\
 	file.go\
 	getwd.go\