anchor.diff:  
     observeTest('toString', () {
       var list = toObservable([1, 2, 3]);
  -    var rol = new ReadOnlyObservableList(list);
  +    var rol = factory(list);
       expect(rol.toString(), '[1, 2, 3]');
     });
   
  
positive.code: import 'package:unittest/unittest.dart';
  import '../src/observe_test_utils.dart';
  
  void main() {
    // TODO(jmesserly): need all standard List API tests.
  
    StreamSubscription sub;
  
negative.code: 
      setUp(() {
        list = toObservable([1, 2, 3]);
        rol = factory(list);
        changes = null;
        sub = rol.changes.listen((records) {
          changes = records.where((r) => r.changes(1)).toList();
  