Replace making getInnermostContext public with making a getFolder method

pkg/analysis_server/lib/src/single_context_manager.dart

 // Copyright (c) 2016, 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.src.single_context_manager;
 
 import 'dart:async';
 import 'dart:core';
 import 'dart:math' as math;
 
@@ skipping 129 matching lines @@
   AnalysisContext getContextFor(String path) {
     if (context == null) {
       return null;
     } else if (_isContainedIn(includedPaths, path)) {
       return context;
     }
     return null;
   }
 
   @override
+  Folder getContextFolderFor(String path) {
+    if (isInAnalysisRoot(path)) {
+      return contextFolder;
+    }
+    return null;
+  }
+
+  @override
   AnalysisDriver getDriverFor(String path) {
     throw new UnimplementedError(
         'Unexpected invocation of getDriverFor in SingleContextManager');
   }
 
   @override
   List<AnalysisDriver> getDriversInAnalysisRoot(Folder analysisRoot) {
     throw new UnimplementedError(
         'Unexpected invocation of getDriversInAnalysisRoot in SingleContextManager');
   }
@@ skipping 358 matching lines @@
   static List<Resource> _getChildrenSafe(Folder folder) {
     try {
       return folder.getChildren();
     } on FileSystemException {
       // The folder either doesn't exist or cannot be read.
       // Either way, there are no children.
       return const <Resource>[];
     }
   }
 }