Add basic support for a configuration file.

lib/src/runner/configuration/values.dart

+// Copyright (c) 2016, 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 'dart:math' as math;
+
+import '../../util/io.dart';
+
+/// The default number of test suites to run at once.
+///
+/// This defaults to half the available processors, since presumably some of
+/// them will be used for the OS and other processes.
+final defaultConcurrency = math.max(1, Platform.numberOfProcessors ~/ 2);
+
+/// The reporters supported by the test runner.
+const allReporters = const ["compact", "expanded", "json"];
+
+/// The default reporter.
+final defaultReporter = inTestTests
+    ? 'expanded'
+    : (Platform.isWindows ? 'expanded' : 'compact');