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

sdk/lib/_internal/pub/lib/src/validator/lib.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 pub.validator.lib;
 
-import 'dart:async' hide TimeoutException;
+import 'dart:async';
 
 import 'package:path/path.dart' as path;
 
 import '../entrypoint.dart';
 import '../io.dart';
 import '../validator.dart';
 
 // TODO(nweiz): When issue 7196 is fixed, complain about non-Dart files in lib.
 /// A validator that checks that libraries in "lib/" (and not "lib/src/") exist
 /// and are well-formed.
@@ skipping 18 matching lines @@
         errors.add('You must have a non-empty "lib" directory.\n'
             "Without that, users cannot import any code from your package.");
       } else if (files.length == 1 && files.first == "src") {
         errors.add('The "lib" directory must contain something other than '
             '"src".\n'
             "Otherwise, users cannot import any code from your package.");
       }
     });
   }
 }