dart:io | Add cache and getters to Platform

sdk/lib/io/platform.dart

 // Copyright (c) 2012, 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.
 
 part of dart.io;
 
 /**
  * The [Platform] class exposes details of the machine and operating
  * system.
  */
 class Platform {
+  static final _numberOfProcessors = _Platform.numberOfProcessors;
+  static final _pathSeparator = _Platform.pathSeparator;
+  static final _operatingSystem = _Platform.operatingSystem;
+  static final _localHostname = _Platform.localHostname;
+
   /**
    * Get the number of processors of the machine.
    */
-  static int get numberOfProcessors => _Platform.numberOfProcessors;
+  static int get numberOfProcessors => _numberOfProcessors;
 
   /**
    * Get the path separator used by the operating system to separate
    * components in file paths.
    */
-  static String get pathSeparator => _Platform.pathSeparator;
+  static String get pathSeparator => _pathSeparator;
 
   /**
-   * Get a string ('macos', 'windows', 'linux') representing the
-   * operating system.
+   * Get a string (`linux`, `macos`, `windows` or `android`)
+   * representing the operating system.
    */
-  static String get operatingSystem => _Platform.operatingSystem;
+  static String get operatingSystem => _operatingSystem;
 
   /**
    * Get the local hostname for the system.
    */
-  static String get localHostname => _Platform.localHostname;
+  static String get localHostname => _localHostname;
+
+  /**
+   * Returns true if the operating system is Linux.
+   */
+  static bool get isLinux => _operatingSystem == "linux";
+
+  /**
+   * Returns true if the operating system is Mac OS.
+   */
+  static bool get isMacOS => _operatingSystem == "macos";
+
+  /**
+   * Returns true if the operating system is Windows.
+   */
+  static bool get isWindows => _operatingSystem == "windows";
+
+  /**
+   * Returns true if the operating system is Android.
+   */
+  static bool get isAndroid => _operatingSystem == "android";
 
   /**
    * Get the environment for this process.
    *
    * Environment variables on Windows are case-insensitive. The map
    * returned on Windows is therefore case-insensitive and will convert
    * all keys to upper case. On other platforms the returned map is
    * a standard case-sensitive map.
    */
   static Map<String, String> get environment => _Platform.environment;
 }