Implement fromEnvironment on bool, int and String

sdk/lib/core/bool.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.core;
 
 /**
  * The reserved words [:true:] and [:false:] denote objects that are the only
  * instances of this class.
  *
  * It is a compile-time error for a class to attempt to extend or implement
  * bool.
  */
 class bool {
-  factory bool._uninstantiable() {
-    throw new UnsupportedError(
-        "class bool cannot be instantiated");
-  }
+  /**
+   * Returns the boolean for the given environment variable [name] or
+   * [defaultValue] if [name] is not present.
+   */
+  external const factory bool.fromEnvironment(String name, {bool defaultValue});
 
   /**
    * Returns [:"true":] if the receiver is [:true:], or [:"false":] if the
    * receiver is [:false:].
    */
   String toString() {
     return this ? "true" : "false";
   }
 }