[element model] migrate resolve_for_completion_test

Change-Id: If66ff0130a8c1015ccf46b287bed2dbbf8a9ca73
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/399782
Auto-Submit: Phil Quitslund <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>

Author: pq

pkg/analyzer/analyzer_use_new_elements.txt

@@ -179,7 +179,6 @@ test/id_tests/type_constraint_generation_test.dart
 test/src/dart/analysis/driver_caching_test.dart
 test/src/dart/analysis/driver_test.dart
 test/src/dart/analysis/index_test.dart
-test/src/dart/analysis/resolve_for_completion_test.dart
 test/src/dart/analysis/result_printer.dart
 test/src/dart/analysis/results/get_element_declaration_test.dart
 test/src/dart/analysis/search_test.dart

pkg/analyzer/test/src/dart/analysis/resolve_for_completion_test.dart

@@ -347,7 +347,7 @@ typedef F^
 extension ResolvedForCompletionResultImplExtension
     on ResolvedForCompletionResultImpl {
   void assertResolvedNodes(List<String> expected) {
-    expect(parsedUnit.declaredElement, isNotNull);
+    expect(parsedUnit.declaredFragment?.element, isNotNull);
 
     var actual = resolvedNodes.map((e) => '$e').toList();
     expect(actual, expected);