Deal with special cmd.exe environment variables that have keys that start with an equals sign.

runtime/bin/process_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.
 
 class _ProcessStartStatus {
   int _errorCode;  // Set to OS error code if process start failed.
   String _errorMessage;  // Set to OS error message if process start failed.
 }
 
 
@@ skipping 46 matching lines @@
       var env = options.environment;
       if (env is !Map) {
         throw new IllegalArgumentException("Environment is not a map: $env");
       }
       _environment = [];
       env.forEach((key, value) {
         if (key is !String || value is !String) {
           throw new IllegalArgumentException(
               "Environment key or value is not a string: ($key, $value)");
         }
-        if (key.contains('=')) {
-          throw new IllegalArgumentException(
-              "Environment keys may not contain '=': $key");
-        }
         _environment.add('$key=$value');
       });
     }
 
     _in = new _Socket._internalReadOnly();  // stdout coming from process.
     _out = new _Socket._internalWriteOnly();  // stdin going to process.
     _err = new _Socket._internalReadOnly();  // stderr coming from process.
     _exitHandler = new _Socket._internalReadOnly();
     _closed = false;
     _ended = false;
@@ skipping 313 matching lines @@
 
 class _ProcessResult implements ProcessResult {
   const _ProcessResult(int this.exitCode,
                        String this.stdout,
                        String this.stderr);
 
   final int exitCode;
   final String stdout;
   final String stderr;
 }