Only start http server for browser tests.

tools/testing/dart/http_server.dart

 // Copyright (c) 2012, 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:io');
 #import('dart:isolate');
 #import('test_suite.dart');  // For TestUtils.
 
@@ skipping 28 matching lines @@
           resp.outputStream.close();
         }
       });
     }
   };
 
   _httpServer.listen(host, port);
 }
 
 terminateHttpServer() {
-  _httpServer.close();
+  if (_httpServer != null) _httpServer.close();
 }