Make my pkg packages warning-clean.

pkg/scheduled_test/lib/src/descriptor/directory_descriptor.dart

Index: pkg/scheduled_test/lib/src/descriptor/directory_descriptor.dart
diff --git a/pkg/scheduled_test/lib/src/descriptor/directory_descriptor.dart b/pkg/scheduled_test/lib/src/descriptor/directory_descriptor.dart
index 5d7b28af15dda5bbe83d09d8290c436d0b3f38a6..f5a2f4ffa83bd71a52f2273dc879a0762120710d 100644
--- a/pkg/scheduled_test/lib/src/descriptor/directory_descriptor.dart
+++ b/pkg/scheduled_test/lib/src/descriptor/directory_descriptor.dart
@@ -85,9 +85,10 @@ class DirectoryDescriptor extends Descriptor implements LoadableDescriptor {
       } 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));
         }
       }
     }));