Check PackageMapUriResolver constructor arguments.

pkg/analysis_server/lib/src/analysis_server.dart

 // Copyright (c) 2014, 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 analysis.server;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:math' show max;
 
@@ skipping 1155 matching lines @@
     }
   }
 
   /**
    * Set up a [SourceFactory] that resolves packages using the given
    * [packageUriResolver].
    */
   SourceFactory _createSourceFactory(UriResolver packageUriResolver) {
     List<UriResolver> resolvers = <UriResolver>[
         new DartUriResolver(analysisServer.defaultSdk),
-        new ResourceUriResolver(resourceProvider),
-        packageUriResolver];
+        new ResourceUriResolver(resourceProvider)];
+    if (packageUriResolver != null) {
+      resolvers.add(packageUriResolver);
+    }
     return new SourceFactory(resolvers);
   }
 }
 
 
 /**
  * A class used by [AnalysisServer] to record performance information
  * such as request latency.
  */
 class ServerPerformance {
@@ skipping 34 matching lines @@
           new DateTime.now().millisecondsSinceEpoch -
           request.clientRequestTime;
       requestLatency += latency;
       maxLatency = max(maxLatency, latency);
       if (latency > 150) {
         ++slowRequestCount;
       }
     }
   }
 }