Fix exception in stack trace from issue 23348

pkg/analysis_server/test/analysis_server_test.dart

Index: pkg/analysis_server/test/analysis_server_test.dart
diff --git a/pkg/analysis_server/test/analysis_server_test.dart b/pkg/analysis_server/test/analysis_server_test.dart
index 0576d89e26466a79dd4ab9513fef5841d551a0ce..d64ea388ec51c289e6baddbb0c0eb410ed63cd34 100644
--- a/pkg/analysis_server/test/analysis_server_test.dart
+++ b/pkg/analysis_server/test/analysis_server_test.dart
@@ -243,6 +243,18 @@ import "../foo/foo.dart";
     expect(source.fullName, filePath);
   }
 
+  test_getContextSourcePair_nonFile() {
+    String dirPath = '/dir';
+    Folder dir = resourceProvider.newFolder(dirPath);
+
+    AnalysisContext context = AnalysisEngine.instance.createAnalysisContext();
+    _configureSourceFactory(context);
+    server.folderMap[dir] = context;
+
+    ContextSourcePair pair = server.getContextSourcePair(dirPath);
+    expect(pair, isNull);
+  }
+
   test_getContextSourcePair_package_inRoot() {
     String rootPath = '/my_package';
     String filePath = rootPath + '/lib/file.dart';