Add error checking for the immutable annotation (issue 27750)

pkg/analyzer/lib/src/dart/element/element.dart

Index: pkg/analyzer/lib/src/dart/element/element.dart
diff --git a/pkg/analyzer/lib/src/dart/element/element.dart b/pkg/analyzer/lib/src/dart/element/element.dart
index 0636e28a842d47da4bfd8b3cd183b25a2d2b20da..cb194b0261d713bb450c24e07da64898b70516fb 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -2465,6 +2465,12 @@ class ElementAnnotationImpl implements ElementAnnotation {
   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";
@@ -2584,6 +2590,12 @@ class ElementAnnotationImpl implements ElementAnnotation {
       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 &&