Support for @JS() validation (#26225).

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 860c02963b5f2e77774e88a8a0a4811f1881cd27..a320b8d79ca9f78424a421fea5e87804f3101afd 100644
--- a/pkg/analyzer/lib/dart/element/element.dart
+++ b/pkg/analyzer/lib/dart/element/element.dart
@@ -624,6 +624,11 @@ abstract class Element implements AnalysisTarget {
   bool get isDeprecated;
 
   /**
+   * Return `true` if this element has an annotation of the form '@JS(..)'.
+   */
+  bool get isJS;
+
+  /**
    * Return `true` if this element has an annotation of the form '@override'.
    */
   bool get isOverride;
@@ -813,6 +818,12 @@ abstract class ElementAnnotation implements ConstantEvaluationTarget {
   bool get isDeprecated;
 
   /**
+   * Return `true` if this annotation marks the associated element with the `JS`
+   * annotation.
+   */
+  bool get isJS;
+
+  /**
    * Return `true` if this annotation marks the associated member as requiring
    * overriding methods to call super.
    */