Show implicit pub dependency on barback in errors.

sdk/lib/_internal/pub/test/test_pub.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.
 
 /// Test infrastructure for testing pub. Unlike typical unit tests, most pub
 /// tests are integration tests that stage some stuff on the file system, run
 /// pub, and then validate the results. This library provides an API to build
 /// tests like that.
 library test_pub;
 
@@ skipping 129 matching lines @@
     });
   }, 'starting a server serving:\n${baseDir.describe()}');
 }
 
 /// Closes [_server]. Returns a [Future] that will complete after the [_server]
 /// is closed.
 Future _closeServer() {
   if (_server == null) return new Future.value();
   var future = _server.close();
   _server = null;
+  _hasServer = false;
   _portCompleterCache = null;
   return future;
 }
 
 /// `true` if the current test spins up an HTTP server.
 bool _hasServer = false;
 
 /// The [d.DirectoryDescriptor] describing the server layout of `/api/packages`
 /// on the test server.
 ///
@@ skipping 698 matching lines @@
   bool matches(item, Map matchState) {
     if (item is! Pair) return false;
     return _firstMatcher.matches(item.first, matchState) &&
         _lastMatcher.matches(item.last, matchState);
   }
 
   Description describe(Description description) {
     description.addAll("(", ", ", ")", [_firstMatcher, _lastMatcher]);
   }
 }