Add error checking for the immutable annotation (issue 27750)

pkg/analyzer/lib/src/dart/element/element.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.element.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 2447 matching lines @@
    */
   static String _DEPRECATED_VARIABLE_NAME = "deprecated";
 
   /**
    * The name of the top-level variable used to mark a method as being a
    * factory.
    */
   static String _FACTORY_VARIABLE_NAME = "factory";
 
   /**
+   * The name of the top-level variable used to mark a class and its subclasses
+   * as being immutable.
+   */
+  static String _IMMUTABLE_VARIABLE_NAME = "immutable";
+
+  /**
    * The name of the class used to JS annotate an element.
    */
   static String _JS_CLASS_NAME = "JS";
 
   /**
    * The name of `js` library, used to define JS annotations.
    */
   static String _JS_LIB_NAME = "js";
 
   /**
@@ skipping 99 matching lines @@
     return false;
   }
 
   @override
   bool get isFactory =>
       element is PropertyAccessorElement &&
       element.name == _FACTORY_VARIABLE_NAME &&
       element.library?.name == _META_LIB_NAME;
 
   @override
+  bool get isImmutable =>
+      element is PropertyAccessorElement &&
+      element.name == _IMMUTABLE_VARIABLE_NAME &&
+      element.library?.name == _META_LIB_NAME;
+
+  @override
   bool get isJS =>
       element is ConstructorElement &&
       element.enclosingElement.name == _JS_CLASS_NAME &&
       element.library?.name == _JS_LIB_NAME;
 
   @override
   bool get isMustCallSuper =>
       element is PropertyAccessorElement &&
       element.name == _MUST_CALL_SUPER_VARIABLE_NAME &&
       element.library?.name == _META_LIB_NAME;
@@ skipping 6510 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }