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

pkg/analysis_server/test/physical_resource_provider_test.dart

Index: pkg/analysis_server/test/physical_resource_provider_test.dart
diff --git a/pkg/analysis_server/test/physical_resource_provider_test.dart b/pkg/analysis_server/test/physical_resource_provider_test.dart
index 63f0a0d484941cdc82e10ff5d2a083f93b74d52d..e16fb82fe4c39994dd6aded9d1690c56993ba402 100644
--- a/pkg/analysis_server/test/physical_resource_provider_test.dart
+++ b/pkg/analysis_server/test/physical_resource_provider_test.dart
@@ -160,6 +160,12 @@ main() {
         test('toString', () {
           expect(file.toString(), path);
         });
+
+        test('parent', () {
+          Resource parent = file.parent;
+          expect(parent, new isInstanceOf<Folder>());
+          expect(parent.path, equals(tempPath));
+        });
       });
 
       group('Folder', () {
@@ -216,6 +222,26 @@ main() {
           expect(children[1], _isFolder);
           expect(children[2], _isFile);
         });
+
+        test('parent', () {
+          Resource parent = folder.parent;
+          expect(parent, new isInstanceOf<Folder>());
+          expect(parent.path, equals(tempPath));
+
+          // Since the OS is in control of where tempPath is, we don't know how
+          // far it should be from the root.  So just verify that each call to
+          // parent results in a a folder with a shorter path, and that we
+          // reach the root eventually.
+          while (true) {
+            Resource grandParent = parent.parent;
+            if (grandParent == null) {
+              break;
+            }
+            expect(grandParent, new isInstanceOf<Folder>());
+            expect(grandParent.path.length, lessThan(parent.path.length));
+            parent = grandParent;
+          }
+        });
       });
     });
 }