Fix issue with Link.target and Link.targetSync failing on some file systems

tests/standalone/io/regress_21987_test.dart

+// Copyright (c) 2015, 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.
+
+import 'dart:io';
+
+import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+
+void main() {
+  if (Platform.isLinux) {
+    asyncStart();
+    var selfExe = new Link('/proc/self/exe');
+    Expect.isTrue(selfExe.targetSync().length > 0);
+    selfExe.target().then((target) {
+      Expect.isTrue(target.length > 0);
+      asyncEnd();
+    });
+  }
+}