Fix write to sink after close in test code.

test/discovery_test.dart

 // Copyright (c) 2015, 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 package_config.discovery_test;
 
 import "dart:async";
 import "dart:io";
 import "package:test/test.dart";
 import "package:package_config/packages.dart";
@@ skipping 272 matching lines @@
           var path = request.uri.path;
           if (path.startsWith('/')) path = path.substring(1);
           if (path.endsWith('/')) path = path.substring(0, path.length - 1);
           var parts = path.split('/');
           var fileOrDir = description;
           for (int i = 0; i < parts.length; i++) {
             fileOrDir = fileOrDir[parts[i]];
             if (fileOrDir == null) {
               request.response.statusCode = 404;
               request.response.close();
+              return;
             }
           }
           request.response.write(fileOrDir);
           request.response.close();
         });
       });
     });
     tearDown(() => serverSub.cancel());
     test(name, () => httpTest(uri));
   });
 }
 
 void generalTest(String name, Map description, Future action(Uri location)) {
   fileTest(name, description, action);
   httpTest(name, description, action);
 }
 
 void _createFiles(Directory target, Map description) {
   description.forEach((name, content) {
     if (content is Map) {
       Directory subDir = new Directory(path.join(target.path, name));
       subDir.createSync();
       _createFiles(subDir, content);
     } else {
       File file = new File(path.join(target.path, name));
       file.writeAsStringSync(content, flush: true);
     }
   });
 }