bug327: document what's being tested

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

test/fixedbugs/bug327.go

@@ -4,15 +4,21 @@
 // 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 (
-       a interface{}
+	a interface{}
-       b interface{}
+	b interface{}
 )
 
 func main() {
-       x := a(1)
+	x := a(1)
-       z := b(x)
+	z := b(x)
-       _ = z
+	_ = z
 }