Remove AsyncError with Expando.

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 "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:io';
 
 testStartError() {
   Future<Process> processFuture =
       Process.start("__path_to_something_that_should_not_exist__",
                     const []);
   processFuture.then((p) => Expect.fail('got process despite start error'))
-  .catchError((e) {
-    Expect.isTrue(e.error is ProcessException);
-    Expect.equals(2, e.error.errorCode, e.error.toString());
+  .catchError((error) {
+    Expect.isTrue(error is ProcessException);
+    Expect.equals(2, error.errorCode, error.toString());
   });
 }
 
 testRunError() {
   Future<ProcessResult> processFuture =
       Process.run("__path_to_something_that_should_not_exist__",
                   const []);
 
   processFuture.then((result) => Expect.fail("exit handler called"))
-  .catchError((e) {
-    Expect.isTrue(e.error is ProcessException);
-    Expect.equals(2, e.error.errorCode, e.error.toString());
+  .catchError((error) {
+    Expect.isTrue(error is ProcessException);
+    Expect.equals(2, error.errorCode, error.toString());
   });
 }
 
 main() {
   testStartError();
   testRunError();
 }