Add support for evaluating platform selectors.

lib/src/runner/test_platform.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.
-
-// TODO(nweiz): support pluggable platforms.
-/// An enum of all platforms on which tests can run.
-class TestPlatform {
-  /// The command-line Dart VM.
-  static const vm = const TestPlatform._("VM", "vm");
-
-  /// Google Chrome.
-  static const chrome = const TestPlatform._("Chrome", "chrome");
-
-  /// A list of all instances of [TestPlatform].
-  static const all = const [vm, chrome];
-
-  /// Finds a platform by its identifier string.
-  ///
-  /// If no platform is found, returns `null`.
-  static TestPlatform find(String identifier) =>
-      all.firstWhere((platform) => platform.identifier == identifier,
-          orElse: () => null);
-
-  /// The human-friendly name of the platform.
-  final String name;
-
-  /// The identifier used to look up the platform.
-  final String identifier;
-
-  const TestPlatform._(this.name, this.identifier);
-
-  String toString() => name;
-}