Fix tests for VM in checked mode.

tests/standalone/io/test_extension_fail_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.
 //
 // Dart test program for testing native extensions.
 
+import "dart:async";
 import "dart:io";
 
 Future copyFileToDirectory(Path file, Path directory) {
   String src = file.toNativePath();
   String dst = directory.toNativePath();
   switch (Platform.operatingSystem) {
     case 'linux':
     case 'macos':
       return Process.run('cp', [src, dst]);
     case 'windows':
@@ skipping 37 matching lines @@
   }).then((_) {
     Path script = testDirectory.append('test_extension_fail_tester.dart');
     return Process.run(options.executable, [script.toNativePath()]);
   }).then((ProcessResult result) {
     print("ERR: ${result.stderr}\n\n");
     print("OUT: ${result.stdout}\n\n");
     Expect.equals(255, result.exitCode);
     Expect.isTrue(result.stderr.contains("Unhandled exception:\nball\n"));
   }).whenComplete(() => tempDirectory.deleteSync(recursive: true));
 }