anchor.diff:    /// than or equal to the given value, using the given orderBy directive or
     /// priority as default, and optionally only child nodes with a key less
     /// than or equal to the given key.
  -  Query endAt(dynamic value, { String key }) {
  +  Query endAt(dynamic value, {String key}) {
       assert(!_parameters.containsKey('endAt'));
       assert(value is String || value is bool || value is double || value is int);
  -    final Map<String, dynamic> parameters = <String, dynamic>{ 'endAt': value };
  +    final Map<String, dynamic> parameters = <String, dynamic>{'endAt': value};
       if (key != null) parameters['endAtKey'] = key;
       return _copyWithParameters(parameters);
     }
  
positive.code:   /// `value` (and `key`, if provided).
    ///
    /// If a key is provided, there is at most one such child as names are unique.
    Query equalTo(dynamic value, { String key }) {
      assert(!_parameters.containsKey('equalTo'));
      assert(value is String || value is bool || value is double || value is int);
      return _copyWithParameters(
        <String, dynamic>{ 'equalTo': value, 'equalToKey': key },
      );
    }
  
    /// Create a query with limit and anchor it to the start of the window.
    Query limitToFirst(int limit) {
      assert(!_parameters.containsKey('limitToFirst'));
      return _copyWithParameters(<String, dynamic>{ 'limitToFirst': limit });
    }
  
    /// Create a query with limit and anchor it to the end of the window.
    Query limitToLast(int limit) {
      assert(!_parameters.containsKey('limitToLast'));
      return _copyWithParameters(<String, dynamic>{ 'limitToLast': limit });
    }
  
    /// Generate a view of the data sorted by values of a particular child key.
  
negative.code:         _handle.then((int handle) async {
            await _database._channel.invokeMethod(
              'Query#removeObserver',
              <String, dynamic>{'handle': handle},
            );
            FirebaseDatabase._observers.remove(handle);
          });
  