Use string hash for sharding tests instead of using an index in a sorted list.

tools/testing/dart/test_suite.dart

Index: tools/testing/dart/test_suite.dart
===================================================================
--- tools/testing/dart/test_suite.dart	(revision 5977)
+++ tools/testing/dart/test_suite.dart	(working copy)
@@ -245,22 +245,6 @@
         // 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
@@ -396,6 +380,14 @@
              bool hasFatalTypeErrors = false,
              bool hasRuntimeErrors = false,
              Set<String> multitestOutcome = null]) {
+      int shards = configuration['shards'];
+      if (shards > 1) {
+        int shard = configuration['shard'];
+        if (filename.hashCode() % shards != shard -1) {
+          return;
+        }
+      }
+
       // Cache the test information for each test case.
       var info = new TestInformation(filename,
                                      optionsFromFile,
@@ -405,10 +397,7 @@
                                      hasRuntimeErrors,
                                      multitestOutcome);
       cachedTests.add(info);
-      if (configuration['shards'] == 1) {
-        // If we are not sharding, we queue the tests as we find them.
-        enqueueTestCaseFromTestInformation(info);
-      }
+      enqueueTestCaseFromTestInformation(info);
     };
   }
 
@@ -891,24 +880,6 @@
              "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;
-  }  
 }