Add iOS as an operating system.

lib/src/backend/operating_system.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.
 
 /// 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", "windows");
 
   /// Mac OS X.
   static const macOS = const OperatingSystem._("OS X", "mac-os");
 
   /// GNU/Linux.
   static const linux = const OperatingSystem._("Linux", "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", "android");
 
+  /// iOS.
+  ///
+  /// Since this is the operating system the test runner is running on, this
+  /// won't be true when testing remotely on an iOS browser.
+  static const iOS = const OperatingSystem._("iOS", "ios");
+
   /// 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", "none");
 
   /// A list of all instances of [OperatingSystem] other than [none].
-  static const all = const [windows, macOS, linux, android];
+  static const all = const [windows, macOS, linux, android, iOS];
 
   /// 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;
+      case "ios": return iOS;
       default: return none;
     }
   }
 
   /// The human-friendly of the operating system.
   final String name;
 
   /// The identifier used to look up the operating system.
   final String identifier;
 
   /// Whether this is a POSIX-ish operating system.
   bool get isPosix => this != windows && this != none;
 
   const OperatingSystem._(this.name, this.identifier);
 
   String toString() => name;
 }