Introduce common superclass to fix ClassElement.node (issue 23120)

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

Index: pkg/analyzer/lib/src/generated/element.dart
diff --git a/pkg/analyzer/lib/src/generated/element.dart b/pkg/analyzer/lib/src/generated/element.dart
index ed87c1fa7bd3a93fc593e0f1a4121dfa83f67e99..120ff4ac8db92ef15e69f2469f5844731671ef86 100644
--- a/pkg/analyzer/lib/src/generated/element.dart
+++ b/pkg/analyzer/lib/src/generated/element.dart
@@ -225,7 +225,7 @@ abstract class ClassElement implements Element {
    * so parsing and resolving will be performed.
    */
   @override
-  AstNode get node;
+  NamedCompilationUnitMember get node;
 
   /**
    * Return the superclass of this class, or `null` if the class represents the
@@ -701,7 +701,7 @@ class ClassElementImpl extends ElementImpl implements ClassElement {
   }
 
   @override
-  AstNode get node {
+  NamedCompilationUnitMember get node {
     if (isEnum) {
       return getNodeMatching((node) => node is EnumDeclaration);
     } else {