Replace the existing unittest APIs with the new runner infrastructure.

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)));
 }
 
 /// 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=${p.join(packageDir, 'packages')}")
-     ..addAll(args);
+  var allArgs = [
+    p.join(packageDir, 'bin/unittest.dart'),
+    "--package-root=${p.join(packageDir, 'packages')}"
+  ]..addAll(args);
+
+  // TODO(nweiz): Use ScheduledProcess once it's compatible.
+  return runDart(allArgs, workingDirectory: workingDirectory);
+}
+
+/// Runs Dart.
+ProcessResult runDart(List<String> args, {String workingDirectory}) {
+  var allArgs = Platform.executableArguments.toList()..addAll(args);
 
   // TODO(nweiz): Use ScheduledProcess once it's compatible.
   return Process.runSync(Platform.executable, allArgs,
       workingDirectory: workingDirectory);
 }