Add read-only environment variable access.

runtime/bin/platform.dart

 // Copyright (c) 2011, 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.
 
 /**
  * The [Platform] class exposes details of the machine and operating
  * system.
  */
 interface Platform default _Platform {
   /**
@@ skipping 15 matching lines @@
   /**
    * Get a string ('macos', 'windows', 'linux') representing the
    * operating system.
    */
   String operatingSystem();
 
   /**
    * Get the local hostname for the system.
    */
   String localHostname();
+
+  /**
+   * Get the environment for this process.
+   */
+  Map<String, String> environment();
 }