[element model] migrate display_string_test

Change-Id: I37dd444ee7de3463d2a40f44d6625cbb88489160
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/401967
Commit-Queue: Phil Quitslund <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: pq

pkg/analyzer/analyzer_use_new_elements.txt

@@ -168,7 +168,6 @@ test/src/dart/analysis/session_test.dart
 test/src/dart/ast/element_locator_test.dart
 test/src/dart/constant/evaluation_test.dart
 test/src/dart/constant/value_test.dart
-test/src/dart/element/display_string_test.dart
 test/src/dart/element/element_test.dart
 test/src/dart/element/function_type_test.dart
 test/src/dart/element/generic_inferrer_test.dart