Report hint when target of an invocation uses '?.', so can be null.

pkg/analyzer/lib/src/generated/error.dart

 // Copyright (c) 2014, 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.
 
 library engine.error;
 
 import 'dart:collection';
 
 import 'package:analyzer/src/task/model.dart';
 import 'package:analyzer/task/model.dart';
@@ skipping 2914 matching lines @@
    *
    * Parameters:
    * 0: the name of the actual argument type
    * 1: the name of the expected type
    */
   static const HintCode ARGUMENT_TYPE_NOT_ASSIGNABLE = const HintCode(
       'ARGUMENT_TYPE_NOT_ASSIGNABLE',
       "The argument type '{0}' cannot be assigned to the parameter type '{1}'");
 
   /**
+   * When the target expression uses '?.' operator, it can be `null`, so all the
+   * subsequent invocations should also use '?.' operator.
+   */
+  static const HintCode CAN_BE_NULL_AFTER_NULL_AWARE = const HintCode(
+      'CAN_BE_NULL_AFTER_NULL_AWARE',
+      "The expression uses '?.', so can be 'null'",
+      "Replace the '.' with a '?.' in the invocation");
+
+  /**
    * Dead code is code that is never reached, this can happen for instance if a
    * statement follows a return statement.
    */
   static const HintCode DEAD_CODE = const HintCode('DEAD_CODE', "Dead code");
 
   /**
    * Dead code is code that is never reached. This case covers cases where the
    * user has catch clauses after `catch (e)` or `on Object catch (e)`.
    */
   static const HintCode DEAD_CODE_CATCH_FOLLOWING_CATCH = const HintCode(
@@ skipping 2152 matching lines @@
    * Initialize a newly created error code to have the given [name].
    */
   const TodoCode(String name) : super(name, "{0}");
 
   @override
   ErrorSeverity get errorSeverity => ErrorSeverity.INFO;
 
   @override
   ErrorType get type => ErrorType.TODO;
 }