Add error checking for the immutable annotation (issue 27750)

pkg/analyzer/lib/src/dart/error/hint_codes.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 analyzer.src.dart.error.hint_codes;
 
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 
 /**
@@ skipping 198 matching lines @@
    * object.
    *
    * Parameters:
    * 0: the name of the method
    */
   static const HintCode INVALID_FACTORY_METHOD_IMPL = const HintCode(
       'INVALID_FACTORY_METHOD_IMPL',
       "Factory method '{0}' doesn't return a newly allocated object.");
 
   /**
+   * This hint is generated anywhere an @immutable annotation is associated with
+   * anything other than a class.
+   */
+  static const HintCode INVALID_IMMUTABLE_ANNOTATION = const HintCode(
+      'INVALID_IMMUTABLE_ANNOTATION',
+      "Only classes can be annotated as being immutable.");
+
+  /**
    * Generic Method DEP: number of type parameters must match.
    * <https://github.com/leafpetersen/dep-generic-methods/blob/master/proposal.md#function-subtyping>
    *
    * Parameters:
    * 0: the number of type parameters in the method
    * 1: the number of type parameters in the overridden method
    * 2: the name of the class where the overridden method is declared
    */
   static const HintCode INVALID_METHOD_OVERRIDE_TYPE_PARAMETERS = const HintCode(
       'INVALID_METHOD_OVERRIDE_TYPE_PARAMETERS',
@@ skipping 109 matching lines @@
    * Parameters:
    * 0: the name of the declared return type
    */
   static const HintCode MISSING_RETURN = const HintCode(
       'MISSING_RETURN',
       "This function declares a return type of '{0}', but doesn't end with a "
       "return statement.",
       "Try adding a return statement, or changing the return type to 'void'.");
 
   /**
+   * Generate a hint for classes that inherit from classes annotated with
+   * `@immutable` but that are not immutable.
+   */
+  static const HintCode MUST_BE_IMMUTABLE = const HintCode(
+      'MUST_BE_IMMUTABLE',
+      "This class inherits from a class marked as @immutable, "
+      "and therefore should be immutable (all fields must be final).");
+
+  /**
    * Generate a hint for methods that override methods annotated `@mustCallSuper`
    * that do not invoke the overridden super method.
    *
    * Parameters:
    * 0: the name of the class declaring the overridden method
    */
   static const HintCode MUST_CALL_SUPER = const HintCode(
       'MUST_CALL_SUPER',
       "This method overrides a method annotated as @mustCallSuper in '{0}', "
       "but does not invoke the overridden method.");
@@ skipping 276 matching lines @@
    */
   const HintCode(String name, String message, [String correction])
       : super(name, message, correction);
 
   @override
   ErrorSeverity get errorSeverity => ErrorType.HINT.severity;
 
   @override
   ErrorType get type => ErrorType.HINT;
 }