Fix an analyzer error in scheduled_test.

pkg/scheduled_test/lib/src/descriptor/descriptor.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.descriptor;
 
 import 'dart:async';
 
 import '../utils.dart';
 
@@ skipping 22 matching lines @@
 
   /// An unscheduled version of [validate]. This is useful if validation errors
   /// need to be caught, since otherwise they'd be registered by the schedule.
   Future validateNow([String parent]);
 
   /// Returns a detailed tree-style description of [this].
   String describe();
 }
 
 /// An interface for descriptors that can load the contents of sub-descriptors.
-abstract class LoadableDescriptor extends Descriptor {
+abstract class LoadableDescriptor implements Descriptor {
   /// Treats [this] as an in-memory filesystem and returns a stream of the
   /// contents of the child entry located at [path]. This only works if [this]
   /// is a directory entry. This operation is not [schedule]d.
   ///
   /// This method uses POSIX paths regardless of the underlying operating
   /// system.
   ///
   /// All errors in loading the file will be passed through the returned
   /// [Stream].
   Stream<List<int>> load(String pathToLoad);
 }
 
 /// An interface for descriptors whose contents can be read.
-abstract class ReadableDescriptor extends Descriptor {
+abstract class ReadableDescriptor implements Descriptor {
   /// Returns the contents of [this] as a stream. This only works if [this] is a
   /// file entry. This operation is not [schedule]d.
   ///
   /// All errors in loading the file will be passed through the returned
   /// [Stream].
   Stream<List<int>> read();
 }