Make my pkg packages warning-clean.

pkg/scheduled_test/lib/src/descriptor/directory_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.directory;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:pathos/path.dart' as path;
@@ skipping 67 matching lines @@
       var adjective = requiresReadable ? 'readable' : 'loadable';
       if (matchingEntries.length == 0) {
         throw "Couldn't find a $adjective entry named '${split.first}' within "
             "'$name'.";
       } else if (matchingEntries.length > 1) {
         throw "Found multiple $adjective entries named '${split.first}' within "
             "'$name'.";
       } else {
         var remainingPath = split.sublist(1);
         if (remainingPath.isEmpty) {
-          return matchingEntries.first.read();
+          return (matchingEntries.first as ReadableDescriptor).read();
         } else {
-          return matchingEntries.first.load(_path.joinAll(remainingPath));
+          return (matchingEntries.first as LoadableDescriptor)
+              .load(_path.joinAll(remainingPath));
         }
       }
     }));
   }
 
   String describe() {
     if (contents.isEmpty) return name;
 
     var buffer = new StringBuffer();
     buffer.writeln(name);
@@ skipping 25 matching lines @@
 
   _DirectoryValidationError(Iterable errors)
       : errors = errors.map((e) => e.toString()).toList();
 
   String toString() {
     if (errors.length == 1) return errors.single;
     return errors.map((e) => prefixLines(e, prefix: '  ', firstPrefix: '* '))
         .join('\n');
   }
 }