Fix error in test.dart where a process will be closed twice.

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 533 matching lines @@
       testCase.completed();
     }
   }
 
   /**
    * Process exit handler called at the end of every command. It internally
    * treats all but the last command as compilation steps. The last command is
    * the actual test and its output is analyzed in [testComplete].
    */
   void stepExitHandler(int exitCode) {
-    process.close();
+    try {
+      process.close();
+    } catch (ProcessException e) {
+      // If the process is already closed, continue.
+    }
     int totalSteps = testCase.commands.length;
     String suffix =' (step $currentStep of $totalSteps)';
     if (currentStep == totalSteps) { // done with test command
       testComplete(exitCode);
     } else if (exitCode != 0) {
       stderr.add('test.dart: Compilation failed$suffix, exit code $exitCode\n');
       testComplete(exitCode);
     } else {
       stderr.add('test.dart: Compilation finished $suffix\n');
       stdout.add('test.dart: Compilation finished $suffix\n');
@@ skipping 574 matching lines @@
         // the developer doesn't waste his or her time trying to fix a bunch of
         // tests that appear to be broken but were actually just flakes that
         // didn't get retried because there had already been one failure.
         bool allowRetry = _MAX_FAILED_NO_RETRY > _progress.numFailedTests;
         new RunningProcess(test, allowRetry, this).start();
       }
       _numProcesses++;
     }
   }
 }