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

pkg/analyzer/lib/file_system/physical_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 physical_file_system;
 
 import 'dart:async';
 import 'dart:io' as io;
 
 import 'package:analyzer/src/generated/java_io.dart';
@@ skipping 55 matching lines @@
 
   @override
   Stream<WatchEvent> get changes => new DirectoryWatcher(_entry.path).events;
 
   @override
   String canonicalizePath(String relPath) {
     return normalize(join(_entry.absolute.path, relPath));
   }
 
   @override
+  bool contains(String path) {
+    return posix.isWithin(this.path, path);
+  }
+
+  @override
   Resource getChild(String relPath) {
     String canonicalPath = canonicalizePath(relPath);
     return PhysicalResourceProvider.INSTANCE.getResource(canonicalPath);
   }
 
   @override
   List<Resource> getChildren() {
     List<Resource> children = <Resource>[];
     io.Directory directory = _entry as io.Directory;
     List<io.FileSystemEntity> entries = directory.listSync(recursive: false);
@@ skipping 44 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }