internal/api_classic.go
func fromContext(ctx netcontext.Context) appengine.Context {
	c, _ := ctx.Value(&contextKey).(appengine.Context)
	return c
}
func fromContext(ctx netcontext.Context) appengine.Context {
	c, _ := ctx.Value(&contextKey).(appengine.Context)
	return c
}
// fromContext returns the App Engine context or nil if ctx is not
// derived from an App Engine context.