When using content_shell to run tests, run them with '--single-process'

tools/testing/dart/test_suite.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 enumerating and preparing tests.
  *
  * This library includes:
  *
  * - Creating tests by listing all the Dart files in certain directories,
@@ skipping 1033 matching lines @@
           if (runtime != "drt") {
             print("Unknown runtime $runtime");
             exit(1);
           }
 
           var dartFlags = [];
           var contentShellOptions = [];
 
           contentShellOptions.add('--no-timeout');
           contentShellOptions.add('--dump-render-tree');
+          contentShellOptions.add('--single-process');
 
           if (compiler == 'none' || compiler == 'dart2dart') {
             dartFlags.add('--ignore-unrecognized-flags');
             if (configuration["checked"]) {
               dartFlags.add('--enable_asserts');
               dartFlags.add("--enable_type_checks");
             }
             dartFlags.addAll(vmOptions);
           }
 
@@ skipping 902 matching lines @@
  * $pass tests are expected to pass
  * $failOk tests are expected to fail that we won't fix
  * $fail tests are expected to fail that we should fix
  * $crash tests are expected to crash that we should fix
  * $timeout tests are allowed to timeout
  * $compileErrorSkip tests are skipped on browsers due to compile-time error
 """;
     print(report);
    }
 }