Very simple version of Isolates.

tests/standalone/io/process_shell_test.dart

 // Copyright (c) 2013, 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.
 
 import "package:path/path.dart";
+import "package:async_helper/async_helper.dart";
 import "dart:io";
+import "dart:async";
 import "dart:isolate";
 
 void testRunShell() {
   test(args) {
+    asyncStart();
     var path = join(dirname(Platform.script), "process_echo_util.dart");
     Process.run(Platform.executable,
                 [path]..addAll(args),
                 runInShell: true)
         .then((result) {
           if (Platform.operatingSystem == "windows") {
             result = result.stdout.split("\r\n");
           } else {
             result = result.stdout.split("\n");
           }
           if (result.length - 1 != args.length) {
             throw "wrong number of args: $args vs $result";
           }
           for (int i = 0; i < args.length; i++) {
             if (args[i] != result[i]) {
               throw "bad result at $i: '${args[i]}' vs '${result[i]}'";
             }
           }
+          asyncEnd();
         });
   }
   test(["\""]);
   test(["a b"]);
   test(["'"]);
   test(["'", "'"]);
   test(["'\"\"'\"'\"'"]);
   test(["'\"\"'", "\"'\"'"]);
   test(["'\\\"\\\"'\\", "\"\\'\"'"]);
   test(["'\$HOME'"]);
   test(["'\$tmp'"]);
   test(["arg'"]);
   test(["arg\\'", "'\\arg"]);
 }
 
 void testBadRunShell() {
   test(exe, [args = const []]) {
+    asyncStart();
     Process.run(exe, args, runInShell: true)
         .then((result) {
-          port.close();
           if (result.exitCode == 0) {
             throw "error expected";
           }
+          asyncEnd();
         });
   }
   test("'\"'");
   test("'\$HOME'");
 }
 
 void main() {
   testRunShell();
   testBadRunShell();
 }