Make test.dart strong mode clean!

tools/testing/dart/http_server.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 http_server;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'dart:convert' show HtmlEscape;
@@ skipping 442 matching lines @@
 """);
     response.close();
     response.done.catchError((e) {
       DebugLogger.warning(
           'HttpServer: error while closing the response stream', e);
     });
   }
 }
 
 // Helper class for displaying directory listings.
-class _Entry implements Comparable {
+class _Entry implements Comparable<_Entry> {
   final String name;
   final String displayName;
 
   _Entry(this.name, this.displayName);
 
   int compareTo(_Entry other) {
     return name.compareTo(other.name);
   }
 }