go/build: avoid os.Error in tests

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5308061

src/pkg/go/build/build_test.go

@@ -28,7 +28,7 @@ var buildPkgs = []struct {
 			GoFiles:      []string{"pkgtest.go"},
 			SFiles:       []string{"sqrt_" + runtime.GOARCH + ".s"},
 			Package:      "pkgtest",
-			Imports:      []string{"os"},
+			Imports:      []string{"bytes"},
 			TestImports:  []string{"fmt", "pkgtest"},
 			TestGoFiles:  sortstr([]string{"sqrt_test.go", "sqrt_" + runtime.GOARCH + "_test.go"}),
 			XTestGoFiles: []string{"xsqrt_test.go"},

src/pkg/go/build/pkgtest/pkgtest.go

@@ -4,9 +4,9 @@
 
 package pkgtest
 
-import "os"
+import "bytes"
 
-func Foo() os.Error {
+func Foo() *bytes.Buffer {
 	return nil
 }