Add Folder.contains(path) and use it in ContextDirectoryManager.

pkg/analyzer/lib/file_system/file_system.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 file_system;
 
 import 'dart:async';
 
 import 'package:analyzer/src/generated/source.dart';
 import 'package:path/path.dart';
@@ skipping 25 matching lines @@
    * If the path [path] is a relative path, convert it to an absolute path
    * by interpreting it relative to this folder.  If it is already an aboslute
    * path, then don't change it.
    *
    * However, regardless of whether [path] is relative or absolute, normalize
    * it by removing path components of the form '.' or '..'.
    */
   String canonicalizePath(String path);
 
   /**
+   * Return `true` if absolute [path] references a resource in this folder.
+   */
+  bool contains(String path);
+
+  /**
    * Return an existing child [Resource] with the given [relPath].
    * Return a not existing [File] if no such child exist.
    */
   Resource getChild(String relPath);
 
   /**
    * Return a list of existing direct children [Resource]s (folders and files)
    * in this folder, in no particular order.
    */
   List<Resource> getChildren();
@@ skipping 71 matching lines @@
   }
 
   /**
    * Return `true` if the given URI is a `file` URI.
    *
    * @param uri the URI being tested
    * @return `true` if the given URI is a `file` URI
    */
   static bool _isFileUri(Uri uri) => uri.scheme == _FILE_SCHEME;
 }