reflect: allow unexported key in Value.MapIndex

Fixes #1748.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4444087

src/pkg/reflect/value.go

@@ -958,14 +958,19 @@ func (v Value) MapIndex(key Value) Value {
 	iv.mustBe(Map)
 	typ := iv.typ.toType()
 
+	// Do not require ikey to be exported, so that DeepEqual
+	// and other programs can use all the keys returned by
+	// MapKeys as arguments to MapIndex.  If either the map
+	// or the key is unexported, though, the result will be
+	// considered unexported.
+
 	ikey := key.internal()
-	ikey.mustBeExported()
 	ikey = convertForAssignment("reflect.Value.MapIndex", nil, typ.Key(), ikey)
 	if iv.word == 0 {
 		return Value{}
 	}
 
-	flag := iv.flag & flagRO
+	flag := (iv.flag | ikey.flag) & flagRO
 	elemType := typ.Elem()
 	elemWord, ok := mapaccess(iv.word, ikey.word)
 	if !ok {