Very simple version of Isolates.

utils/testrunner/testrunner.dart

 //#!/usr/bin/env 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.
 
 /**
  * testrunner is a program to run Dart unit tests. Unlike $DART/tools/test.dart,
  * this program is intended for 3rd parties to be able to run unit tests in
  * a batched fashion. As such, it adds some features and removes others. Some
  * of the removed features are:
@@ skipping 238 matching lines @@
       port.close();
       --_numTasks;
       if (exitCode == 0 || !config['stop-on-failure']) {
         spawnTasks(config, testFiles);
       }
       if (_numTasks == 0) {
         // No outstanding tasks; we're all done.
         // We could later print a summary report here.
       }
     });
-    SendPort s = spawnUri(config['pipeline']);
-
     // Get the names of the source and target test files and containing
     // directories.
     var testPath = new Path(testfile);
     var sourcePath = testPath.directoryPath;
     var sourceDir = sourcePath.toNativePath();
 
     var targetPath = new Path(config["tempdir"]);
     var normalizedTarget = testPath.directoryPath.toNativePath()
         .replaceAll(Platform.pathSeparator, '_')
         .replaceAll(':', '_');
     targetPath = targetPath.append("${normalizedTarget}_${config['runtime']}");
     var targetDir = targetPath.toNativePath();
 
     config['targetDir'] = targetDir;
     // If this is a new target dir, we need to redo the pub check.
     var f = null;
     if (targetDir != _testDir) {
       f = doPubConfig(sourcePath, sourceDir, targetPath, targetDir,
           config['pub'], config['runtime']);
       _testDir = targetDir;
     }
-    if (f == null) {
-      s.send(config, port.toSendPort());
-    } else {
-      f.then((_) {
-        s.send(config, port.toSendPort());
-      });
-      break; // Don't do any more until pub is done.
-    }
+    var response = new ReceivePort();
+    spawnUri(config['pipeline'], [], response)
+        .then((_) => f)
+        .then((_) => response.first)
+        .then((s) { s.send([config, port.sendPort]); });
+    if (f != null) break; // Don't do any more until pub is done.
   }
 }
 
 /**
  * Our tests are configured so that critical messages have a '###' prefix.
  * [writelog] takes the output from a pipeline execution and writes it to
  * our output sinks. It will strip the '###' if necessary on critical
  * messages; other messages will only be written if verbose output was
  * specified.
  */
@@ skipping 117 matching lines @@
       if (dirs.length == 0) {
         dirs.add('.'); // Use current working directory as default.
       }
       var f = buildFileList(dirs,
           new RegExp(config['test-file-pattern']), config['recurse']);
       if (config['sort']) f.sort();
       processTests(config, f);
     }
   }
 }