fix observe package to be strong mode clean

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 'dart:async';
 import 'package:observe/observe.dart';
 import 'package:observe/src/path_observer.dart'
     show getSegmentsOfPropertyPathForTesting,
          observerSentinelForTesting;
 
@@ skipping 143 matching lines @@
     expect(new PathObserver(null, '').value, null);
     expect(new PathObserver(123, '').value, 123);
     expect(() => new PathObserver(123, 'foo.bar.baz').value, _throwsNSM('foo'));
 
     // shouldn't throw:
     new PathObserver(123, '')..open((_) {})..close();
     new PropertyPath('').setValueFrom(null, null);
     new PropertyPath('').setValueFrom(123, 42);
     expect(() => new PropertyPath('foo.bar.baz').setValueFrom(123, 42),
         _throwsNSM('foo'));
-    var foo = {};
+    Object foo = {};
     expect(new PathObserver(foo, '').value, foo);
 
     foo = new Object();
     expect(new PathObserver(foo, '').value, foo);
 
     expect(new PathObserver(foo, 'a/3!').value, null);
   });
 
   test('get value at path ObservableBox', () {
     var obj = new ObservableBox(new ObservableBox(new ObservableBox(1)));
@@ skipping 529 matching lines @@
     if (index == 'foo') return _foo;
   }
 
   operator []=(index, value) {
     log.add('[]= $index $value');
     if (index == 'foo') _foo = value;
   }
 }
 
 @reflectable
-class TestModel extends ChangeNotifier {
+class TestModel extends ChangeNotifier implements WatcherModel {
   var _a, _b, _c;
 
   TestModel([this._a, this._b, this._c]);
 
   get a => _a;
 
   void set a(newValue) {
     _a = notifyPropertyChange(#a, _a, newValue);
   }
 
@@ skipping 18 matching lines @@
   @observable var c;
 
   WatcherModel();
 }
 
 class Foo {
   var value;
   Foo(this.value);
   String toString() => 'Foo$value';
 }