Add a Resource.parent getter, which gets the parent directory.

pkg/analysis_server/test/resource_test.dart

Index: pkg/analysis_server/test/resource_test.dart
diff --git a/pkg/analysis_server/test/resource_test.dart b/pkg/analysis_server/test/resource_test.dart
index 489741d5e80fb685b6d9a557fd1e0f17f33f3fd1..409e0afd82954f4d8a807bd91d50eb6c404831a7 100644
--- a/pkg/analysis_server/test/resource_test.dart
+++ b/pkg/analysis_server/test/resource_test.dart
@@ -259,6 +259,14 @@ main() {
         File file = provider.getResource('/foo/bar/file.txt');
         expect(file.toString(), '/foo/bar/file.txt');
       });
+
+      test('parent', () {
+        provider.newFile('/foo/bar/file.txt', 'content');
+        File file = provider.getResource('/foo/bar/file.txt');
+        Resource parent = file.parent;
+        expect(parent, new isInstanceOf<Folder>());
+        expect(parent.path, equals('/foo/bar'));
+      });
     });
 
     group('Folder', () {
@@ -311,6 +319,16 @@ main() {
         expect(children[1], _isFolder);
         expect(children[2], _isFile);
       });
+
+      test('parent', () {
+        Resource parent1 = folder.parent;
+        expect(parent1, new isInstanceOf<Folder>());
+        expect(parent1.path, equals('/foo'));
+        Resource parent2 = parent1.parent;
+        expect(parent2, new isInstanceOf<Folder>());
+        expect(parent2.path, equals('/'));
+        expect(parent2.parent, isNull);
+      });
     });
 
     group('_MemoryFileSource', () {