Fix exception in stack trace from issue 23348

pkg/analysis_server/lib/src/analysis_server.dart

Index: pkg/analysis_server/lib/src/analysis_server.dart
diff --git a/pkg/analysis_server/lib/src/analysis_server.dart b/pkg/analysis_server/lib/src/analysis_server.dart
index d468ce5bc5a394dbda24fda0db1486f516a63065..485d3a2f0a1238835392cbb335f57bf6276d25f1 100644
--- a/pkg/analysis_server/lib/src/analysis_server.dart
+++ b/pkg/analysis_server/lib/src/analysis_server.dart
@@ -443,7 +443,10 @@ class AnalysisServer {
     }
     // 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) {