Avoid error in testing script if environment is empty.

tools/testing/dart/test_runner.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.
 
 /**
  * Classes and methods for executing tests.
  *
  * This module includes:
  * - Managing parallel execution of tests, including timeout checks.
  * - Evaluating the output of each test as pass/fail/crash/timeout.
@@ skipping 117 matching lines @@
 
   bool _equal(ProcessCommand other) =>
       super._equal(other) &&
       executable == other.executable &&
       deepJsonCompare(arguments, other.arguments) &&
       workingDirectory == other.workingDirectory &&
       deepJsonCompare(environmentOverrides, other.environmentOverrides);
 
   String get reproductionCommand {
     var env = new StringBuffer();
-    environmentOverrides.forEach((key, value) =>
+    environmentOverrides?.forEach((key, value) =>
         (io.Platform.operatingSystem == 'windows')
             ? env.write('set $key=${escapeCommandLineArgument(value)} & ')
             : env.write('$key=${escapeCommandLineArgument(value)} '));
     var command = ([executable]..addAll(arguments))
         .map(escapeCommandLineArgument)
         .join(' ');
     if (workingDirectory != null) {
       command = "$command (working directory: $workingDirectory)";
     }
     return "$env$command";
@@ skipping 2911 matching lines @@
     }
   }
 
   void eventAllTestsDone() {
     for (var listener in _eventListener) {
       listener.allDone();
     }
     _allDone();
   }
 }