Fix static warnings introduced by not having pid in base CommandOutput class.

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 843 matching lines @@
   bool hasFailed(TestCase testCase);
 
   bool get canRunDependendCommands;
 
   bool get successful; // otherwise we might to retry running
 
   Duration get time;
 
   int get exitCode;
 
+  int get pid;
+
   List<int> get stdout;
 
   List<int> get stderr;
 
   List<String> get diagnostics;
 
   bool get compilationSkipped;
 }
 
 class CommandOutputImpl extends UniqueObject implements CommandOutput {
@@ skipping 1981 matching lines @@
     }
   }
 
   void eventAllTestsDone() {
     for (var listener in _eventListener) {
       listener.allDone();
     }
     _allDone();
   }
 }