test: [a-c]: add introductory comments to tests
Very few of the compiler regression tests include a comment
saying waht they do. Many are obvious, some are anything but.
I've started with a-c in the top directory. More will follow once
we agree on the approach, correctness, and thoroughness here.
zerodivide.go sneaked in too.

R=rsc, r
CC=golang-dev
https://golang.org/cl/5656100
diff --git a/test/alias1.go b/test/alias1.go
index 11c18f8..4219af8 100644
--- a/test/alias1.go
+++ b/test/alias1.go
@@ -4,11 +4,11 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package main
-
 // Test that dynamic interface checks treat byte=uint8
 // and rune=int or rune=int32.
 
+package main
+
 func main() {
 	var x interface{}