Fix the tests from the latest CL.

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 'dart:mirrors';
 
 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)));
-}
+String _computePackageDir() =>
+    p.dirname(p.dirname(_libraryPath(#unittest.test.io)));
 
 /// Runs the unittest executable with the package root set properly.
 ProcessResult runUnittest(List<String> args, {String workingDirectory}) {
   var allArgs = Platform.executableArguments.toList()
      ..add(p.join(packageDir, 'bin/unittest.dart'))
      ..add("--package-root=${p.join(packageDir, 'packages')}")
      ..addAll(args);
 
   // TODO(nweiz): Use ScheduledProcess once it's compatible.
   return Process.runSync(Platform.executable, allArgs,
       workingDirectory: workingDirectory);
 }
+
+/// Returns the path to the library named [libraryName].
+///
+/// The library name must be globally unique, or the wrong library path may be
+/// returned.
+String _libraryPath(Symbol libraryName) {
+  var lib = currentMirrorSystem().findLibrary(libraryName);
+  return p.fromUri(lib.uri);
+}
+
+