Unify dart:async and pub/io.dart TimeoutException classes.

sdk/lib/_internal/pub/test/io_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.
 
 library io_test;
 
-import 'dart:async' hide TimeoutException;
+import 'dart:async';
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
 import 'package:unittest/unittest.dart';
 
 import '../lib/src/io.dart';
 import 'test_pub.dart';
 
 main() {
   initConfig();
@@ skipping 405 matching lines @@
           expect(predicate(symlink2Path), equals(forMultiLevelFileSymlink));
         }), completes);
       });
     }
   });
 }
 
 /// Like [withTempDir], but canonicalizes the path before passing it to [fn].
 Future withCanonicalTempDir(Future fn(String path)) =>
   withTempDir((temp) => fn(canonicalize(temp)));