bug327: document what's being tested

R=r
CC=golang-dev
https://golang.org/cl/4380043

test/fixedbugs/bug327.go

@@ -4,6 +4,12 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Conversion between identical interfaces.
+// Issue 1647.
+
+// The compiler used to not realize this was a no-op,
+// so it generated a call to the non-existent function runtime.convE2E.
+
 package main
 
 type (