oauth2/google: exclude the App Engine implementations if not on an AE app

Change-Id: If44bb4498975495301b85e2cf44f73d3373ab16c
Reviewed-on: https://go-review.googlesource.com/2980
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

google/appenginedoc.go

@@ -0,0 +1,19 @@
+// Copyright 2015 The oauth2 Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !appengine,!appenginevm
+
+package google
+
+import "golang.org/x/oauth2"
+
+// AppEngineTokenSource returns a token source that fetches tokens
+// issued to the current App Engine application's service account.
+// If you are implementing a 3-legged OAuth 2.0 flow on App Engine
+// that involves user accounts, see oauth2.Config instead.
+//
+// You are required to provide a valid appengine.Context as context.
+func AppEngineTokenSource(ctx oauth2.Context, scope ...string) oauth2.TokenSource {
+	panic("You should only use an AppEngineTokenSource in an App Engine application.")
+}