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

tests/standalone/io/process_start_exception_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 errors during startup of the process.
 
 #import("dart:io");
 
 testStartError() {
-  Process process = Process.start("__path_to_something_that_should_not_exist__",
-                                  const []);
-
-  process.onExit = (int exitCode) {
-    Expect.fail("exit handler called");
-  };
-
-  process.onError = (ProcessException e) {
+  Future<Process> processFuture =
+      Process.start("__path_to_something_that_should_not_exist__",
+                    const []);
+  processFuture.then((p) => Expect.fail('got process despite start error'));
+  processFuture.handleException((e) {
+    Expect.isTrue(e is ProcessException);
     Expect.equals(2, e.errorCode, e.toString());
-  };
+    return true;
+  });
 }
 
-
 testRunError() {
   Future<ProcessResult> processFuture =
       Process.run("__path_to_something_that_should_not_exist__",
                   const []);
 
   processFuture.then((result) => Expect.fail("exit handler called"));
 
   processFuture.handleException((e) {
     Expect.isTrue(e is ProcessException);
     Expect.equals(2, e.errorCode, e.toString());
     return true;
   });
 }
 
 main() {
   testStartError();
   testRunError();
 }