Add support for running browser tests to the test runner.

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:unittest/src/util/io.dart';
 
 /// The path to the root directory of the `unittest` package.
 final String packageDir = 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,
     Map<String, String> environment}) {
   var allArgs = [
-    p.join(packageDir, 'bin/unittest.dart'),
+    p.absolute(p.join(packageDir, 'bin/unittest.dart')),
     "--package-root=${p.join(packageDir, 'packages')}"
   ]..addAll(args);
 
   if (environment == null) environment = {};
   environment.putIfAbsent("_UNITTEST_USE_COLOR", () => "false");
 
   // TODO(nweiz): Use ScheduledProcess once it's compatible.
   return runDart(allArgs, workingDirectory: workingDirectory,
       environment: environment);
 }
 
 /// Runs Dart.
 ProcessResult runDart(List<String> args, {String workingDirectory,
     Map<String, String> environment}) {
   var allArgs = Platform.executableArguments.toList()..addAll(args);
 
   // TODO(nweiz): Use ScheduledProcess once it's compatible.
   return Process.runSync(Platform.executable, allArgs,
       workingDirectory: workingDirectory, environment: environment);
 }