Compute the script Uri lazily

sdk/lib/io/platform_impl.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;
 
 class _Platform {
   external static int _numberOfProcessors();
   external static String _pathSeparator();
   external static String _operatingSystem();
@@ skipping 15 matching lines @@
    * A value may contain further '=' characters, and it may be empty.
    *
    * Returns an [OSError] if retrieving the environment fails.
    */
   external static _environment();
   external static List<String> _executableArguments();
   external static String _packageRoot();
   external static String _packageConfig();
   external static String _version();
   external static String _localeName();
+  external static Uri _script();
 
   static String executable = _executable();
   static String resolvedExecutable = _resolvedExecutable();
   static String packageRoot = _packageRoot();
   static String packageConfig = _packageConfig();
 
   static String _cachedLocaleName;
   static String get localeName {
     if (_cachedLocaleName == null) {
       var result = _localeName();
       if (result is OSError) {
         throw result;
       }
       _cachedLocaleName = result;
     }
     return _cachedLocaleName;
   }
 
   // Cache the OS environment. This can be an OSError instance if
   // retrieving the environment failed.
   static var /*OSError|Map<String,String>*/ _environmentCache;
 
   static int get numberOfProcessors => _numberOfProcessors();
   static String get pathSeparator => _pathSeparator();
   static String get operatingSystem => _operatingSystem();
-  static Uri script;
+  static Uri get script => _script();
 
   static String get localHostname {
     var result = _localHostname();
     if (result is OSError) {
       throw result;
     } else {
       return result;
     }
   }
 
@@ skipping 68 matching lines @@
     _map.forEach(f);
   }
 
   Iterable<String> get keys => _map.keys;
   Iterable<V> get values => _map.values;
   int get length => _map.length;
   bool get isEmpty => _map.isEmpty;
   bool get isNotEmpty => _map.isNotEmpty;
   String toString() => _map.toString();
 }