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

pkg/analyzer/test/file_system/physical_resource_provider_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.physical_file_system;
 
 import 'dart:async';
 import 'dart:io' as io;
 
 import 'package:analyzer/file_system/file_system.dart';
@@ skipping 197 matching lines @@
           expect(folder == folder2, isTrue);
         });
 
         test('equality: different paths', () {
           String path2 = join(tempPath, 'folder2');
           new io.Directory(path2).createSync();
           Folder folder2 = PhysicalResourceProvider.INSTANCE.getResource(path2);
           expect(folder == folder2, isFalse);
         });
 
+        test('contains', () {
+          expect(folder.contains(join(path, 'aaa.txt')), isTrue);
+          expect(folder.contains(join(path, 'aaa', 'bbb.txt')), isTrue);
+          expect(folder.contains(join(tempPath, 'baz.txt')), isFalse);
+          expect(folder.contains(path), isFalse);
+        });
+
         group('getChild', () {
           test('does not exist', () {
             var child = folder.getChild('no-such-resource');
             expect(child, isNotNull);
             expect(child.exists, isFalse);
           });
 
           test('file', () {
             new io.File(join(path, 'myFile')).createSync();
             var child = folder.getChild('myFile');
@@ skipping 58 matching lines @@
           expect(folder.canonicalizePath('baz'), equals(join(path, 'baz')));
           expect(folder.canonicalizePath(path2), equals(path2));
           expect(folder.canonicalizePath(join('..', 'folder2')), equals(path2));
           expect(folder.canonicalizePath(join(path2, '..', 'folder3')), equals(path3));
           expect(folder.canonicalizePath(join('.', 'baz')), equals(join(path, 'baz')));
           expect(folder.canonicalizePath(join(path2, '.', 'baz')), equals(join(path2, 'baz')));
         });
       });
     });
 }