Reorganize mdv and observe packages

pkg/observe/test/observe_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:mdv_observe/mdv_observe.dart';
+import 'package:observe/observe.dart';
 import 'package:unittest/unittest.dart';
 import 'utils.dart';
 
 main() {
   // Note: to test the basic Observable system, we use ObservableBox due to its
   // simplicity.
 
   const _VALUE = const Symbol('value');
 
   group('ObservableBox', () {
@@ skipping 104 matching lines @@
           }));
           t.value = 777;
         }));
       }));
       t.value = 42;
     });
   });
 }
 
 _record(key) => new PropertyChangeRecord(key);