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

sdk/lib/_internal/pub/lib/src/source.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.source;
 
-import 'dart:async' hide TimeoutException;
+import 'dart:async';
 
 import 'package:path/path.dart' as path;
 
 import 'io.dart';
 import 'package.dart';
 import 'pubspec.dart';
 import 'system_cache.dart';
 import 'utils.dart';
 import 'version.dart';
 
@@ skipping 247 matching lines @@
   /// Returns the [Package]s that have been downloaded to the system cache.
   List<Package> getCachedPackages() {
     if (shouldCache) {
       throw new UnimplementedError("Source $name must implement this.");
     }
   }
 
   /// Returns the source's name.
   String toString() => name;
 }