Add support for evaluating platform selectors.

lib/src/backend/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.
 
+library unittest.backend.test_platform;
+
 // TODO(nweiz): support pluggable platforms.
 /// An enum of all platforms on which tests can run.
 class TestPlatform {
+  // When adding new platforms, be sure to update the baseline and derived
+  // variable tests in test/backend/platform_selector/evaluate_test.
+
   /// The command-line Dart VM.
-  static const vm = const TestPlatform._("VM", "vm");
+  static const vm = const TestPlatform._("VM", "vm", isDartVm: true);
 
   /// Google Chrome.
-  static const chrome = const TestPlatform._("Chrome", "chrome");
+  static const chrome = const TestPlatform._("Chrome", "chrome",
+      isBrowser: true, isJS: true, isBlink: true);
 
   /// 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);
+  /// Whether this platform runs the Dart VM in any capacity.
+  final bool isDartVm;
+
+  /// Whether this platform is a browser.
+  final bool isBrowser;
+
+  /// Whether this platform runs Dart compiled to JavaScript.
+  final bool isJS;
+
+  /// Whether this platform uses the Blink rendering engine.
+  final bool isBlink;
+
+  const TestPlatform._(this.name, this.identifier, {this.isDartVm: false,
+      this.isBrowser: false, this.isJS: false, this.isBlink: false});
 
   String toString() => name;
 }