Change the process API to be completely asynchronous.

tests/standalone/src/ProcessExitNegativeTest.dart

 // Copyright (c) 2011, 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 compilation errors in the process
 // exit handler is reported correctly.
 
 void main() {
-  Process p = new Process("true", []);
+  Process p = new Process.start("true", []);
   p.exitHandler = (int s) {
     print(a.toString());  // Should cause a compilation error here.
     p.close();
   };
-  p.start();
 }