Add fields to platform library, implement them on runtime dart.

runtime/lib/platform_patch.dart

 // Copyright (c) 2013, 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.
 
+patch int get numberOfProcessors => _platform.numberOfProcessors;
+
+patch String get pathSeparator => _platform.pathSeparator;
+
+patch String get operatingSystem => _platform.operatingSystem;
+
+patch String get localHostname => _platform.localHostname;
+
+patch String get version => _platform.version;
+
+patch Map<String, String> get environment => _platform.environment;
+
+patch String get executable => _platform.executable;
+
+patch Uri get script => _platform.script;
+
+patch List<String> get executableArguments => _platform.executableArguments;
+
+patch String get packageRoot => _platform.packageRoot;
+
+patch bool get isLinux => _platform.operatingSystem == "linux";
+
+patch bool get isMacOS => _platform.operatingSystem == "macos";
+
+patch bool get isWindows => _platform.operatingSystem == "windows";
+
+patch bool get isAndroid => _platform.operatingSystem == "android";
+
+class _Platform {
+  int get numberOfProcessors;
+  String get pathSeparator;
+  String get operatingSystem;
+  String get localHostname;
+  String get version;
+  Map get environment;
+  Uri get script;
+  String get executable;
+  List<String>  get executableArguments;
+  String get packageRoot;
+}
+
+// A non-default _Platform, with real values, is stored here by the embedder.
+_Platform _platform = new _DefaultPlatform();
+
+class _DefaultPlatform implements _Platform {
+  int get numberOfProcessors {
+    return null;
+  }
+
+  String get pathSeparator {
+    return "/";
+  }
+
+  String get operatingSystem {
+    return null;
+  }
+
+  String get localHostname {
+    return null;
+  }
+
+  String get version {
+    return null;
+  }
+
+  Map get environment {
+    return null;
+  }
+
+  Uri get script {
+    return null;
+  }
+
+  String get executable {
+    return null;
+  }
+
+  List<String>  get executableArguments {
+    return new List<String>(0);
+  }
+
+  String get packageRoot {
+    return null;
+  }
+}