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

pkg/analyzer/test/file_system/physical_resource_provider_test.dart

Index: pkg/analyzer/test/file_system/physical_resource_provider_test.dart
diff --git a/pkg/analyzer/test/file_system/physical_resource_provider_test.dart b/pkg/analyzer/test/file_system/physical_resource_provider_test.dart
index a4542638323acca95556ec315809883f5a8acbc2..75bfcb2a31de49a5adb886b4d43464869a4693ac 100644
--- a/pkg/analyzer/test/file_system/physical_resource_provider_test.dart
+++ b/pkg/analyzer/test/file_system/physical_resource_provider_test.dart
@@ -215,6 +215,13 @@ main() {
           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');