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

lib/compiler/implementation/lib/io.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.
 
 // This is a copy of the VM's dart:io library. This API is not usable
 // when running inside a web browser. Nevertheless, Leg provides a
 // mock version of the dart:io library so that it can statically
 // analyze programs that use dart:io.
 
 // TODO(ahe): Separate API from implementation details.
@@ skipping 136 matching lines @@
 }
 
 class _DirectoryLister {
 }
 
 void _exit(int exitCode) {
   throw new UnsupportedOperationException("exit($exitCode)");
 }
 
 class _Process {
-  static Process start(String executable,
-                       List<String> arguments,
-                       [ProcessOptions options]) {
+  static Future<Process> start(String executable,
+                               List<String> arguments,
+                               [ProcessOptions options]) {
     var msg = 'Process.start($executable, $arguments, $options)';
     throw new UnsupportedOperationException(msg);
   }
 
   static Future<ProcessResult> run(String executable,
                                    List<String> arguments,
                                    [ProcessOptions options]) {
     var msg = 'Process.run($executable, $arguments, $options)';
     throw new UnsupportedOperationException(msg);
   }
@@ skipping 33 matching lines @@
   static void set _eventHandler(_EventHandler e) {
     throw new UnsupportedOperationException('_EventHandler._eventhandler = $e');
   }
 }
 
 const InputStream stdin = null;
 
 const OutputStream stdout = null;
 
 const OutputStream stderr = null;