dl: add go1.12beta1

Change-Id: I55b16bbe98171e084de7aefabedb19f473d0c8b1
Reviewed-on: https://go-review.googlesource.com/c/154745
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

go1.12beta1/main.go

@@ -0,0 +1,24 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The go1.12beta1 command runs the go command from Go 1.12beta1.
+//
+// To install, run:
+//
+//     $ go get golang.org/dl/go1.12beta1
+//     $ go1.12beta1 download
+//
+// And then use the go1.12beta1 command as if it were your normal go
+// command.
+//
+// See the release notes at https://tip.golang.org/doc/go1.12
+//
+// File bugs at https://golang.org/issues/new
+package main
+
+import "golang.org/dl/internal/version"
+
+func main() {
+	version.Run("go1.12beta1")
+}