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

tests/standalone/io/process_kill_unstarted_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 that an unstarted process cannot be killed.
-
-#library("ProcessKillUnstartedTest");
-#import("dart:io");
-
-main() {
-  var p = Process.start('________', []);
-  Expect.throws(p.kill, (e) => e is ProcessException);
-  p.onError = (e) => Expect.isTrue(e is ProcessException);
-  p.onStart = () => Expect.fail("Process not expected to start");
-}