Support passing an environment variable map to child processes.

runtime/bin/process.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.
 
 /**
  * [Process] objects are used to start new processes and interact with
  * them.
  */
 interface Process default _Process {
   /**
@@ skipping 123 matching lines @@
   /**
    * The encoding used for text on stderr when starting a
    * non-interactive process with [:Process.run:].
    *
    * This option is ignored for interactive processes started with
    * [:Process.start:].
    *
    * The default stderrEncoding is UTF_8.
    */
   Encoding stderrEncoding;
+
+  /**
+   * Provides the environment variables for the process. If not set
+   * the environment of the parent process is inherited.
+   *
+   * Currently, only ASCII environment variables are supported and
+   * errors are likely to occur if an environment variables with
+   * code-points outside the ASCII range is passed in.
+   */
+  Map<String, String> environment;
 }
 
 
 class ProcessException implements Exception {
   const ProcessException([String this.message, int this.errorCode = 0]);
   String toString() => "ProcessException: $message";
 
   /**
    * Contains the system message for the process exception if any.
    */
   final String message;
 
   /**
    * Contains the OS error code for the process exception if any.
    */
   final int errorCode;
 }