gc: double-check usage of ...

Fixes #423.

R=ken2
https://golang.org/cl/180045
diff --git a/test/fixedbugs/bug228.go b/test/fixedbugs/bug228.go
new file mode 100644
index 0000000..682152e
--- /dev/null
+++ b/test/fixedbugs/bug228.go
@@ -0,0 +1,19 @@
+// errchk $G -e $D/$F.go
+
+// Copyright 2009 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func f(x int, y ...)	// ok
+
+func g(x int, y float) (...)	// ERROR "[.][.][.]"
+
+func h(x, y ...)		// ERROR "[.][.][.]"
+
+func i(x int, y ..., z float)	// ERROR "[.][.][.]"
+
+var x ...;		// ERROR "[.][.][.]|syntax"
+
+type T ...;		// ERROR "[.][.][.]|syntax"