[element model] migrate document_ignores

Bug: dart-lang/linter#5099
Change-Id: Ib2a67357d403b5741ac28b72676334135e03b3d6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/387607
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Phil Quitslund <pquitslund@google.com>

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

@@ -39,7 +39,7 @@ class _Visitor extends SimpleAstVisitor<void> {
 
   @override
   void visitCompilationUnit(CompilationUnit node) {
-    var content = node.declaredElement?.source.contents.data;
+    var content = node.declaredFragment?.source.contents.data;
     for (var comment in node.ignoreComments) {
       var ignoredElements = comment.ignoredElements;
       if (ignoredElements.isEmpty) {