misc/dist: remove exp and old before building

Fixes #3317.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5784074

misc/dist/bindist.go

@@ -42,6 +42,11 @@ const (
 	uploadURL    = "https://go.googlecode.com/files"
 )
 
+var preBuildCleanFiles = []string{
+	"src/pkg/exp",
+	"src/pkg/old",
+}
+
 var cleanFiles = []string{
 	".hg",
 	".hgtags",
@@ -117,6 +122,11 @@ func (b *Build) Do() error {
 		return err
 	}
 
+	// Remove exp and old packages.
+	if err := b.clean(preBuildCleanFiles); err != nil {
+		return err
+	}
+
 	src := filepath.Join(b.root, "src")
 	if b.Source {
 		if runtime.GOOS == "windows" {