Fix exception in stack trace from issue 23348

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 425 matching lines @@
     {
       Uri uri = resourceProvider.pathContext.toUri(path);
       Source sdkSource = defaultSdk.fromFileUri(uri);
       if (sdkSource != null) {
         AnalysisContext anyContext = folderMap.values.first;
         return new ContextSourcePair(anyContext, sdkSource);
       }
     }
     // try to find the deep-most containing context
     Resource resource = resourceProvider.getResource(path);
-    File file = resource is File ? resource : null;
+    if (resource is! File) {
+      return null;

[scheglov, 2015/05/04 15:38:54]

This breaks this method's contract.

1. It promises not to return nulls.
  We need to check for "context != null" anyway, and it is easier to do just
once than first check for "pair != null" and then "context != null".

2. It promises to return a context, even if not a file is given.
   However I think that this is actually never used.

So, we can probably fix the contract and return "new ContextSourcePair(null,
null)" here.

[Brian Wilkerson, 2015/05/04 15:55:58]

I misread the comment. Done.

+    }
+    File file = resource;
     {
       AnalysisContext containingContext = getContainingContext(path);
       if (containingContext != null) {
         Source source = file != null
             ? ContextManager.createSourceInContext(containingContext, file)
             : null;
         return new ContextSourcePair(containingContext, source);
       }
     }
     // try to find a context that analysed the file
     for (AnalysisContext context in folderMap.values) {
       Source source = file != null

[scheglov, 2015/05/04 15:38:54]

Now we can remove all checks of "file" for null.

[Brian Wilkerson, 2015/05/04 15:55:58]

Done

           ? ContextManager.createSourceInContext(context, file)
           : null;
       SourceKind kind = context.getKindOf(source);
       if (kind != SourceKind.UNKNOWN) {
         return new ContextSourcePair(context, source);
       }
     }
     // try to find a context for which the file is a priority source
     for (InternalAnalysisContext context in folderMap.values) {
       List<Source> sources = context.getSourcesWithFullName(path);
@@ skipping 1030 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');
 }