Match gccgo error message.

bug231.go:20:4: error: incompatible types in assignment (type has no methods)

R=rsc
CC=golang-dev
https://golang.org/cl/194156

test/fixedbugs/bug231.go

@@ -17,6 +17,6 @@ func main() {
 	var i I
 	
 	i = m
-	i = t	// ERROR "not a method"
+	i = t	// ERROR "not a method|has no methods"
 	_ = i
 }