Shard tests run by test.dart, so they can be distributed to multiple machines.

tools/testing/dart/test_options.dart

 // Copyright (c) 2011, 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.
 
 #library("test_options_parser");
 
 #import("dart:io");
 #import("dart:builtin");
 #import("drt_updater.dart");
 
@@ skipping 147 matching lines @@
               [],
               false,
               'bool'),
           new _TestOptionSpecification(
               'tasks',
               'The number of parallel tasks to run',
               ['-j', '--tasks'],
               [],
               new Platform().numberOfProcessors(),
               'int'),
           new _TestOptionSpecification(

[Bill Hesse, 2012/03/28 22:46:19]

Since nobody replied to Kasper's suggestion of --shard 4:6, I didn't change the
options.

+              'shards',
+              'The number of instances that the tests will be sharded over',
+              ['--shards'],
+              [],
+              1,
+              'int'),
+          new _TestOptionSpecification(
+              'shard',
+              'The index of this instance when running in sharded mode',
+              ['--shard'],
+              [],
+              1,
+              'int'),
+          new _TestOptionSpecification(
               'help',
               'Print list of options',
               ['-h', '--help'],
               [],
               false,
               'bool'),
           new _TestOptionSpecification(
               'verbose',
               'Verbose output',
               ['-v', '--verbose'],
@@ skipping 170 matching lines @@
     }
 
     // Apply default values for unspecified options.
     for (var option in _options) {
       if (!configuration.containsKey(option.name)) {
         configuration[option.name] = option.defaultValue;
       }
     }
 
     List<Map> expandedConfigs = _expandConfigurations(configuration);
-    return expandedConfigs.filter(_isValidConfig);
+    List<Map> result = expandedConfigs.filter(_isValidConfig);
+    return result.isEmpty() ? null : result;
   }
 
   /**
    * Determine if a particular configuration has a valid combination of compiler
    * and runtime elements.
    */
   bool _isValidConfig(Map config) {
     bool isValid = true;
       switch (config['compiler']) {
         case 'frog':
@@ skipping 15 matching lines @@
     if (!isValid) {
       print("Warning: combination of ${config['compiler']} and " +
           "${config['runtime']} is invalid. Skipping this combination.");
     }
     if (config['runtime'] == 'ie' && 
         new Platform().operatingSystem() != 'windows') {
       isValid = false;
       print("Warning cannot run Internet Explorer on non-Windows operating" +
           " system.");
     }
+    if (config['shard'] < 1 || config['shard'] > config['shards']) {
+      isValid = false;
+      print("Error: shard index is ${config['shard']} out of " + 
+            "${config['shards']} shards");
+    }
     return isValid;
   }
 
   /**
    * Recursively expand a configuration with multiple values per key
    * into a list of configurations with exactly one value per key.
    */
   List<Map> _expandConfigurations(Map configuration) {
     // Expand the pseudo-values such as 'all'.
     if (configuration['arch'] == 'all') {
@@ skipping 189 matching lines @@
         return option;
       }
     }
     print('Unknown test option $name');
     exit(1);
   }
 
 
   List<_TestOptionSpecification> _options;
 }