Only stop HTTP server if it was actually started.

pkg/http/test/utils.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 test_utils;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:json' as json;
 import 'dart:uri';
@@ skipping 90 matching lines @@
         response.contentLength = body.length;
         response.write(body);
         response.close();
       });
     });
   });
 }
 
 /// Stops the current HTTP server.
 void stopServer() {
-  _server.close();
-  _server = null;
+  if (_server != null) {
+    _server.close();
+    _server = null;
+  }
 }
 
 /// Removes eight spaces of leading indentation from a multiline string.
 ///
 /// Note that this is very sensitive to how the literals are styled. They should
 /// be:
 ///     '''
 ///     Text starts on own line. Lines up with subsequent lines.
 ///     Lines are indented exactly 8 characters from the left margin.
 ///     Close is on the same line.'''
@@ skipping 76 matching lines @@
 const isSocketIOException = const _SocketIOException();
 
 /// A matcher for functions that throw SocketIOException.
 const Matcher throwsSocketIOException =
     const Throws(isSocketIOException);
 
 class _SocketIOException extends TypeMatcher {
   const _SocketIOException() : super("SocketIOException");
   bool matches(item, MatchState matchState) => item is SocketIOException;
 }