[observe] add isEmpty/isNotEmpty to ObservableList reflection

pkg/observe/test/path_observer_test.dart

 // Copyright (c) 2013, 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.
 
 import 'package:observe/observe.dart';
 import 'package:unittest/unittest.dart';
 import 'observe_test_utils.dart';
 
 // This file contains code ported from:
 // https://github.com/rafaelw/ChangeSummary/blob/master/tests/test.js
@@ skipping 134 matching lines @@
 
   observeTest('Path Value With Indices', () {
     var model = toObservable([]);
     var path = observePath(model, '0');
     path.changes.listen(expectAsync1((_) {
       expect(path.value, 123);
     }));
     model.add(123);
   });
 
+  group('ObservableList', () {
+    observeTest('isNotEmpty', () {
+      var model = new ObservableList();
+      var path = observePath(model, 'isNotEmpty');
+      expect(path.value, false);
+
+      var future = path.changes.first.then((_) {
+        expect(path.value, true);
+      });
+      model.add(123);
+
+      return future;
+    });
+
+    observeTest('isEmpty', () {
+      var model = new ObservableList();
+      var path = observePath(model, 'isEmpty');
+      expect(path.value, true);
+
+      var future = path.changes.first.then((_) {
+        expect(path.value, false);
+      });
+      model.add(123);
+
+      return future;
+    });
+  });
+
   for (var createModel in [() => new TestModel(), () => new WatcherModel()]) {
     observeTest('Path Observation - ${createModel().runtimeType}', () {
       var model = createModel()..a =
           (createModel()..b = (createModel()..c = 'hello, world'));
 
       var path = observePath(model, 'a.b.c');
       var lastValue = null;
       var sub = path.changes.listen((_) { lastValue = path.value; });
 
       model.a.b.c = 'hello, mom';
@@ skipping 176 matching lines @@
 
 class WatcherModel extends Observable {
   // TODO(jmesserly): dart2js does not let these be on the same line:
   // @observable var a, b, c;
   @observable var a;
   @observable var b;
   @observable var c;
 
   WatcherModel();
 }