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

pkg/analyzer/lib/file_system/memory_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 memory_file_system;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/engine.dart' show TimestampedData;
@@ skipping 272 matching lines @@
 
   @override
   String canonicalizePath(String relPath) {
     relPath = posix.normalize(relPath);
     String childPath = posix.join(path, relPath);
     childPath = posix.normalize(childPath);
     return childPath;
   }
 
   @override
+  bool contains(String path) {
+    return posix.isWithin(this.path, path);
+  }
+
+  @override
   Resource getChild(String relPath) {
     String childPath = canonicalizePath(relPath);
     _MemoryResource resource = _provider._pathToResource[childPath];
     if (resource == null) {
       resource = new _MemoryFile(_provider, childPath);
     }
     return resource;
   }
 
   @override
@@ skipping 40 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }