[element model] migrate eol_at_end_of_file

Bug: ihttps://github.com/dart-lang/linter/issues/5099
Change-Id: Ic5d34042543f561b19a4f73ae17a1d375ba69757
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/387624
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>

Author: pq

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

@@ -36,7 +36,7 @@ class _Visitor extends SimpleAstVisitor<void> {
 
   @override
   void visitCompilationUnit(CompilationUnit node) {
-    var content = node.declaredElement?.source.contents.data;
+    var content = node.declaredFragment?.source.contents.data;
     if (content != null &&
         content.isNotEmpty &&
         // TODO(srawlins): Re-implement this check without iterating over