Fix an analyzer error in scheduled_test.

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

Index: pkg/scheduled_test/lib/src/descriptor/descriptor.dart
diff --git a/pkg/scheduled_test/lib/src/descriptor/descriptor.dart b/pkg/scheduled_test/lib/src/descriptor/descriptor.dart
index e2341182ae29f66b08a80047d90e34094bacc106..82342c45063273353da5b063fb61a8eff1fff422 100644
--- a/pkg/scheduled_test/lib/src/descriptor/descriptor.dart
+++ b/pkg/scheduled_test/lib/src/descriptor/descriptor.dart
@@ -40,7 +40,7 @@ abstract class Descriptor {
 }
 
 /// 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.
@@ -54,7 +54,7 @@ abstract class LoadableDescriptor extends Descriptor {
 }
 
 /// 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.
   ///