Add argument check to Directory and Link constructors

tests/standalone/io/directory_fuzz_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // 'fuzz' test the directory APIs by providing unexpected type
 // arguments. The test passes if the VM does not crash.
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
+import "package:expect/expect.dart";
 
 import 'fuzz_support.dart';
 
 fuzzSyncMethods() {
   typeMapping.forEach((k, v) {
     doItSync(() {
       var d = new Directory(v);
       doItSync(d.existsSync);
       doItSync(d.createSync);
       doItSync(d.deleteSync);
@@ skipping 11 matching lines @@
         doItSync(() => d.listSync(recursive: v2));
       });
     });
   });
 }
 
 fuzzAsyncMethods() {
   var port = new ReceivePort();
   var futures = [];
   typeMapping.forEach((k, v) {
+    if (v is! String) {
+      Expect.throws(() => new Directory(v),
+                    (e) => e is ArgumentError);
+      return;
+    }
     var d = new Directory(v);
     futures.add(doItAsync(d.exists));
     futures.add(doItAsync(d.create));
     futures.add(doItAsync(d.delete));
     futures.add(doItAsync(() {
       return d.createTemp().then((temp) {
         return temp.delete();
       });
     }));
     futures.add(doItAsync(() {
       return d.exists().then((res) {
         if (!res) return d.delete(recursive: true);
         return new Future.value(true);
       });
     }));
     typeMapping.forEach((k2, v2) {
       futures.add(doItAsync(() => d.rename(v2)));
       futures.add(doItAsync(() {
         d.list(recursive: v2).listen((_) {}, onError: (e) => null);
       }));
     });
   });
   Future.wait(futures).then((ignore) => port.close());
 }
 
 main() {
   fuzzSyncMethods();
   fuzzAsyncMethods();
 }