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

sdk/lib/_internal/pub/test/descriptor/tar.dart

 // Copyright (c) 2013, 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 descriptor.tar;
 
-import 'dart:async' hide TimeoutException;
+import 'dart:async';
 
 import 'package:path/path.dart' as path;
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:scheduled_test/descriptor.dart';
 
 import '../../lib/src/io.dart';
 
 /// Describes a tar file and its contents.
 class TarFileDescriptor extends DirectoryDescriptor
     implements ReadableDescriptor {
@@ skipping 26 matching lines @@
     throw new UnimplementedError("TODO(nweiz): implement this");
   }
 
   Stream<List<int>> read() {
     return new Stream<List<int>>.fromFuture(withTempDir((tempDir) {
       return create(tempDir).then((_) =>
           readBinaryFile(path.join(tempDir, name)));
     }));
   }
 }