Get new tests working on dart2js.

test/io.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.
+
+library unittest.test.io;
+
+import 'dart:io';
+
+import 'package:path/path.dart' as p;
+import 'package:stack_trace/stack_trace.dart';
+
+/// The root directory of the `unittest` package.
+final String packageDir = _computePackageDir();
+String _computePackageDir() {
+  var trace = new Trace.current();
+  return p.dirname(p.dirname(p.fromUri(trace.frames.first.uri)));
+}
+
+/// Returns a matcher that matches a [FileSystemException] with the given
+/// [message].
+Matcher isFileSystemException(String message) => predicate(
+    (error) => error is FileSystemException && error.message == message,
+    'is a FileSystemException with message "$message"');
+