Change Process.start to return a future that completes with a

tests/standalone/io/process_exit_test.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.
 //
 // Process test program to test process communication.
 
 #library("ProcessExitTest");
 #import("dart:io");
 #source("process_test_util.dart");
 
 testExit() {
-  Process process = Process.start(getProcessTestFileName(),
-                                  const ["0", "0", "99", "0"]);
-
-  process.onExit = (int exitCode) {
-    Expect.equals(exitCode, 99);
-    process.close();
-  };
+  var future = Process.start(getProcessTestFileName(),
+                             const ["0", "0", "99", "0"]);
+  future.then((process) {
+    process.onExit = (int exitCode) {
+      Expect.equals(exitCode, 99);
+      process.close();
+    };
+  });
 }
 
 testExitRun() {
   Process.run(getProcessTestFileName(),
               const ["0", "0", "99", "0"]).then((result) {
     Expect.equals(result.exitCode, 99);
     Expect.equals(result.stdout, '');
     Expect.equals(result.stderr, '');
   });
 }
 
 main() {
   testExit();
   testExitRun();
 }