Add support for evaluating platform selectors.

lib/src/backend/operating_system.dart

Index: lib/src/backend/operating_system.dart
diff --git a/lib/src/backend/operating_system.dart b/lib/src/backend/operating_system.dart
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+++ b/lib/src/backend/operating_system.dart
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+// 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.operating_system;
+
+/// An enum of all operating systems supported by Dart.
+///
+/// This is used for selecting which operating systems a test can run on. Even
+/// for browser tests, this indicates the operating system of the machine
+/// running the test runner.
+class OperatingSystem {
+  /// Microsoft Windows.
+  static const windows = const OperatingSystem._("windows");
+
+  /// Mac OS X.
+  static const macOS = const OperatingSystem._("mac-os");
+
+  /// GNU/Linux.
+  static const linux = const OperatingSystem._("linux");
+
+  /// Android.
+  ///
+  /// Since this is the operating system the test runner is running on, this
+  /// won't be true when testing remotely on an Android browser.
+  static const android = const OperatingSystem._("android");
+
+  /// No operating system.
+  ///
+  /// This is used when running in the browser, or if an unrecognized operating
+  /// system is used. It can't be referenced by name in platform selectors.
+  static const none = const OperatingSystem._("none");
+
+  /// A list of all instances of [OperatingSystem] other than [none].
+  static const all = const [windows, macOS, linux, android];
+
+  /// Finds an operating system by its name.
+  ///
+  /// If no operating system is found, returns [none].
+  static OperatingSystem find(String name) =>
+      all.firstWhere((platform) => platform.name == name, orElse: () => null);
+
+  /// Finds an operating system by the return value from `dart:io`'s
+  /// `Platform.operatingSystem`.
+  ///
+  /// If no operating system is found, returns [none].
+  static OperatingSystem findByIoName(String name) {
+    switch (name) {
+      case "windows": return windows;
+      case "macos": return macOS;
+      case "linux": return linux;
+      case "android": return android;
+      default: return none;
+    }
+  }
+
+  /// The name of the operating system.
+  final String name;
+
+  /// Whether this is a POSIX-ish operating system.
+  bool get isPosix => this != windows && this != none;
+
+  const OperatingSystem._(this.name);
+
+  String toString() => name;
+}