Add error checking for the immutable annotation (issue 27750)

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

Index: pkg/analyzer/lib/dart/element/element.dart
diff --git a/pkg/analyzer/lib/dart/element/element.dart b/pkg/analyzer/lib/dart/element/element.dart
index ddc60a5ea02866c9b82f46f53238693ff93eae1a..853cf841bf15c5df5b7550d9e991143178935aec 100644
--- a/pkg/analyzer/lib/dart/element/element.dart
+++ b/pkg/analyzer/lib/dart/element/element.dart
@@ -826,6 +826,12 @@ abstract class ElementAnnotation
   bool get isFactory;
 
   /**
+   * Return `true` if this annotation marks the associated class and its
+   * subclasses as being immutable.
+   */
+  bool get isImmutable;
+
+  /**
    * Return `true` if this annotation marks the associated element with the `JS`
    * annotation.
    */
@@ -1277,11 +1283,6 @@ abstract class FunctionElement implements ExecutableElement, LocalElement {
 }
 
 /**
- * The pseudo-declaration that defines a generic function type.
- */
-abstract class GenericFunctionTypeElement implements FunctionTypedElement {}
-
-/**
  * A function type alias (`typedef`).
  *
  * Clients may not extend, implement or mix-in this class.
@@ -1327,6 +1328,11 @@ abstract class FunctionTypedElement implements TypeParameterizedElement {
 }
 
 /**
+ * The pseudo-declaration that defines a generic function type.
+ */
+abstract class GenericFunctionTypeElement implements FunctionTypedElement {}
+
+/**
  * A combinator that causes some of the names in a namespace to be hidden when
  * being imported.
  *