test: add test case for issue 13268

See https://go-review.googlesource.com/#/c/17047/ for the bug fix.

Change-Id: Id5b0a37439d0f8f1e668f7c2ac84335b37f3a300
Reviewed-on: https://go-review.googlesource.com/17177Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/issue13268.go

+// run
+
+// Copyright 2015 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.
+
+// Test error message when EOF is encountered in the
+// middle of a BOM.
+//
+// Since the error requires an EOF, we cannot use the
+// errorcheckoutput mechanism.
+
+package main
+
+import (
+	"io/ioutil"
+	"log"
+	"os"
+	"os/exec"
+	"runtime"
+	"strings"
+)
+
+func main() {
+	// cannot use temp file on nacl via child process
+	if runtime.GOOS == "nacl" {
+		return
+	}
+
+	// create source
+	f, err := ioutil.TempFile("", "issue13268-")
+	if err != nil {
+		log.Fatalf("could not create source file: %v", err)
+	}
+	f.Write([]byte("package p\n\nfunc \xef\xef")) // if this fails, we will die later
+	f.Close()
+	defer os.Remove(f.Name())
+
+	// compile and test output
+	cmd := exec.Command("go", "tool", "compile", f.Name())
+	out, err := cmd.CombinedOutput()
+	if err == nil {
+		log.Fatalf("expected cmd/compile to fail")
+	}
+	if strings.HasPrefix(string(out), "illegal UTF-8 sequence") {
+		log.Fatalf("error %q not found", out)
+	}
+}