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

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 227 matching lines @@
     globalTemporaryDirectory = globalTempDir;
 
     var filesRead = 0;
     void statusFileRead() {
       filesRead++;
       if (filesRead == statusFilePaths.length) {
         // Checked if we have already found and generated the tests for
         // this suite.
         if (!testCache.containsKey(suiteName)) {
           cachedTests = testCache[suiteName] = [];
+          if (configuration['shards'] > 1) {
+            // If we are sharding tests, we do not enqueue them as they are
+            // found in the directory listing.  We add all the tests to the
+            // testCache, then delete all but this shard's tests from the
+            // cache.  The doDone function then enqueues the tests from the
+            // cache, and other configurations also use the shard from the
+            // cache.
+            Function oldDone = doDone;
+            doDone = () {
+              testCache[suiteName] = shardTests();
+              for (var info in testCache[suiteName]) {
+                enqueueTestCaseFromTestInformation(info);
+              }
+              oldDone();
+            };
+          }            
           processDirectory();
         } else {
           // We rely on enqueueing completing asynchronously so use a
           // timer to make it so.
           void enqueueCachedTests(Timer ignore) {
             for (var info in testCache[suiteName]) {
               enqueueTestCaseFromTestInformation(info);
             }
             doDone();
           }
@@ skipping 124 matching lines @@
              Set<String> multitestOutcome = null]) {
       // Cache the test information for each test case.
       var info = new TestInformation(filename,
                                      optionsFromFile,
                                      isNegative,
                                      isNegativeIfChecked,
                                      hasFatalTypeErrors,
                                      hasRuntimeErrors,
                                      multitestOutcome);
       cachedTests.add(info);
-      enqueueTestCaseFromTestInformation(info);
+      if (configuration['shards'] == 1) {
+        // If we are not sharding, we queue the tests as we find them.
+        enqueueTestCaseFromTestInformation(info);
+      }
     };
   }
 
   void processFile(String filename) {
     if (!isTestFile(filename)) return;
 
     // Only run the tests that match the pattern.
     RegExp pattern = configuration['selectors'][suiteName];
     if (!pattern.hasMatch(filename)) return;
     if (filename.endsWith('test_config.dart')) return;
@@ skipping 462 matching lines @@
              "otherScripts": otherScripts,
              "isMultitest": isMultitest,
              "containsLeadingHash": containsLeadingHash,
              "isolateStubs": isolateStubs,
              "containsDomImport": containsDomImport,
              "isLibraryDefinition": isLibraryDefinition,
              "containsSourceOrImport": containsSourceOrImport,
              "numStaticTypeAnnotations": numStaticTypeAnnotations,
              "numCompileTimeAnnotations": numCompileTimeAnnotations};
   }
+
+  /**
+   * shardTests takes the list of tests, stored as the List<TestInformation>
+   * cachedTests, and selects only the tests belonging to this shard.
+   * The tests are sorted by filename, and if there are n shards and we are
+   * shard number i, only the tests at indices equal to i-1 modulo n are kept.
+   */
+  List<TestInformation> shardTests() {
+    cachedTests.sort((TestInformation a, TestInformation b) => a.filename.compareTo(b.filename));
+    int n = configuration['shards'];
+    int i = configuration['shard'];
+    if (n >= 2) {
+      int current = 0;
+      // The test function given to cachedTests.filter uses the entry's index.
+      cachedTests = cachedTests.filter((t) => ((++current % n) == i - 1));
+    }
+    return cachedTests;
+  }  
 }
 
 
 class DartcCompilationTestSuite extends StandardTestSuite {
   List<String> _testDirs;
   int activityCount = 0;
 
   DartcCompilationTestSuite(Map configuration,
                             String suiteName,
                             String directoryPath,
@@ skipping 346 matching lines @@
  * $noCrash tests are expected to be flaky but not crash
  * $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
 """;
     print(report);
    }
 }