test: added test case for (fixed) issue 15470

Follow-up to https://golang.org/cl/22543.

Change-Id: I873b4fa6616ac2aea8faada2fccd126233bbc07f
Reviewed-on: https://go-review.googlesource.com/22583
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/issue15470.dir/a.go

+package a
+
+import "io"
+
+type T interface {
+	M0(_ int)
+	M1(x, _ int) // _ (blank) caused crash
+	M2() (x, _ int)
+}
+
+type S struct{}
+
+func (S) M0(_ int) {}
+func (S) M1(x, _ int) {}
+func (S) M2() (x, _ int) { return }
+func (_ S) M3() {}
+
+// Snippet from x/tools/godoc/analysis/analysis.go.
+// Offending code from #5470.
+type Link interface {
+	Start() int
+	End() int
+	Write(w io.Writer, _ int, start bool) // _ (blank) caused crash
+}

test/fixedbugs/issue15470.dir/b.go

+package b
+
+import _ "./a" // must not fail

test/fixedbugs/issue15470.go

+// compiledir
+
+// Copyright 2016 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.
+
+// Issue 15470: Make sure special-case signatures can
+// be exported and imported w/o problems.
+
+package ignored