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

pkg/analyzer/test/file_system/memory_file_system_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.memory_file_system;
 
 import 'dart:async';
 
 import 'package:analyzer/src/generated/engine.dart' show TimestampedData;
 import 'package:analyzer/src/generated/source.dart';
@@ skipping 285 matching lines @@
         Folder folder2 = provider.getResource(path);
         expect(folder == folder2, isTrue);
       });
 
       test('equality: different paths', () {
         String path2 = '/foo/baz';
         Folder folder2 = provider.newFolder(path2);
         expect(folder == folder2, isFalse);
       });
 
+      test('contains', () {
+        expect(folder.contains('/foo/bar/aaa.txt'), isTrue);
+        expect(folder.contains('/foo/bar/aaa/bbb.txt'), isTrue);
+        expect(folder.contains('/baz.txt'), isFalse);
+        expect(folder.contains('/foo/bar'), isFalse);
+      });
+
       group('getChild', () {
         test('does not exist', () {
           File file = folder.getChild('file.txt');
           expect(file, isNotNull);
           expect(file.exists, isFalse);
         });
 
         test('file', () {
           provider.newFile('/foo/bar/file.txt', 'content');
           File child = folder.getChild('file.txt');
@@ skipping 153 matching lines @@
         });
 
         test('resolveRelative', () {
           Uri relative = source.resolveRelativeUri(new Uri.file('bar/baz.dart'));
           expect(relative.path, '/foo/bar/baz.dart');
         });
       });
     });
   });
 }