Elements. Migrate DeclarationHelper.

Change-Id: I4558c53ba01d19ca3db8ae9903b3759bf3393b9e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/399022
Reviewed-by: Phil Quitslund <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analysis_server/analyzer_use_new_elements.txt

@@ -26,7 +26,6 @@ lib/src/operation/operation_analysis.dart
 lib/src/protocol_server.dart
 lib/src/search/element_references.dart
 lib/src/search/type_hierarchy.dart
-lib/src/services/completion/dart/declaration_helper.dart
 lib/src/services/correction/namespace.dart
 lib/src/services/correction/util.dart
 lib/src/services/kythe/kythe_visitors.dart