Add support for running browser tests to the test runner.

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;
+}