reflect fixes

R=r
https://golang.org/cl/156104

src/pkg/reflect/type.go

@@ -569,6 +569,8 @@ func (t *StructType) NumField() int	{ return len(t.fields) }
 // Same memory layouts, different method sets.
 func toType(i interface{}) Type {
 	switch v := i.(type) {
+	case nil:
+		return nil
 	case *runtime.BoolType:
 		return (*BoolType)(unsafe.Pointer(v))
 	case *runtime.DotDotDotType:

src/pkg/reflect/value.go

@@ -906,7 +906,10 @@ func setiface(typ *InterfaceType, x *interface{}, addr addr)
 
 // Set assigns x to v.
 func (v *InterfaceValue) Set(x Value) {
-	i := x.Interface();
+	var i interface{}
+	if x != nil {
+		i = x.Interface()
+	}
 	if !v.canSet {
 		panic(cannotSet)
 	}

src/pkg/runtime/reflect.cgo

@@ -108,5 +108,11 @@ func setiface(typ *byte, x *byte, ret *byte) {
 		*(Eface*)ret = *(Eface*)x;
 		return;
 	}
+	if(((Eface*)x)->type == nil) {
+		// can assign nil to any interface
+		((Iface*)ret)->tab = nil;
+		((Iface*)ret)->data = nil;
+		return;
+	}
 	ifaceE2I((InterfaceType*)gettype(typ), *(Eface*)x, (Iface*)ret);
 }