[element model] fix diagnostic_describe_all_properties

Bug: https://github.com/dart-lang/linter/issues/5099
Change-Id: Ib3e52fbf1e8698e33fa5d9e33d87bff6e266009a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/390380
Auto-Submit: Phil Quitslund <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: pq

pkg/linter/lib/src/rules/diagnostic_describe_all_properties.dart

@@ -74,7 +74,7 @@ class _Visitor extends SimpleAstVisitor<void> {
   @override
   void visitClassDeclaration(ClassDeclaration node) {
     // We only care about Diagnosticables.
-    var type = node.declaredElement?.thisType;
+    var type = node.declaredFragment?.element.thisType;
     if (!type.implementsInterface('Diagnosticable', '')) return;
 
     var properties = <Token>[];