Allow file resolver provider to return null, just like package resolver provider

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:core' hide Resource;
 import 'dart:io' as io;
@@ skipping 1695 matching lines @@
         embedderSdk.analysisOptions = options;
         // TODO(brianwilkerson) Enable summary use after we have decided where
         // summary files for embedder files will live.
         embedderSdk.useSummary = false;
         return embedderSdk;
       });
       resolvers.add(new DartUriResolver(dartSdk));
     }
 
     resolvers.addAll(packageUriResolvers);
-    if (context.fileResolverProvider == null) {
-      resolvers.add(new ResourceUriResolver(resourceProvider));
-    } else {
-      resolvers.add(context.fileResolverProvider(folder));
+    UriResolver fileResolver;
+    if (context.fileResolverProvider != null) {
+      fileResolver = context.fileResolverProvider(folder);
     }
+    resolvers.add(fileResolver ?? new ResourceUriResolver(resourceProvider));
     return new SourceFactory(resolvers, disposition.packages);
   }
 }
 
 /**
  * A class used by [AnalysisServer] to record performance information
  * such as request latency.
  */
 class ServerPerformance {
   /**
@@ skipping 85 matching lines @@
   /**
    * The [PerformanceTag] for time spent in server request handlers.
    */
   static PerformanceTag serverRequests = new PerformanceTag('serverRequests');
 
   /**
    * The [PerformanceTag] for time spent in split store microtasks.
    */
   static PerformanceTag splitStore = new PerformanceTag('splitStore');
 }