Test that interfaces work in type switches.

R=ken,rsc
DELTA=30  (30 added, 0 deleted, 0 changed)
OCL=26599
CL=26604

test/bugs/bug141.go

+// $G $D/$F.go && $L $F.$A && ./$A.out || echo BUG: should run
+
+// 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
+
+type S struct { i int }
+func (p *S) Get() int { return p.i }
+
+type Empty interface {
+}
+
+type Getter interface {
+	Get() int;
+}
+
+func f1(p Empty) {
+	switch x := p.(type) {
+	default: println("failed to match interface"); sys.Exit(1);
+	case Getter: break;
+	}
+
+}
+
+func main() {
+	var s S;
+	f1(&s);
+}

test/golden.out

@@ -141,6 +141,10 @@ bugs/bug140.go:6: syntax error near L1
 bugs/bug140.go:7: syntax error near L2
 BUG should compile
 
+=========== bugs/bug141.go
+failed to match interface
+BUG: should run
+
 =========== fixedbugs/bug016.go
 fixedbugs/bug016.go:7: constant -3 overflows uint