Fix exception in stack trace from issue 23348

pkg/analysis_server/test/analysis_server_test.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 test.analysis_server;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/constants.dart';
@@ skipping 225 matching lines @@
     server.folderMap[dir2] = context2;
 
     ContextSourcePair pair = server.getContextSourcePair(filePath);
     Source source = pair.source;
     expect(pair.context, same(context2));
     expect(source, isNotNull);
     expect(source.uri.scheme, 'file');
     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';
     Folder rootFolder = resourceProvider.newFolder(rootPath);
     resourceProvider.newFile(filePath, 'library lib;');
 
     packageMapProvider.packageMap = <String, List<Folder>>{
       'my_package': <Folder>[rootFolder]
     };
 
@@ skipping 225 matching lines @@
   _MockServerOperation(this.context);
 
   @override
   ServerOperationPriority get priority => ServerOperationPriority.ANALYSIS;
 
   @override
   void perform(AnalysisServer server) {
     isComplete = true;
   }
 }