Create LSP CompletionItem from CandidateSuggestion, bypassing the SuggestionBuilder.

This eliminates the step of converting CandidateSuggestion to legacy completion item and then into LSP completion item. Also makes use of the SuggestionCollector to sort and truncate suggestions.

The utility functions/methods are in completion_utils.dart, these are based on the code in mapping.dart, modified to use CandidateSuggestions.

Change-Id: I1e44a5318575eeb3b89803e20a082f98b7bedd41
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/392082
Commit-Queue: Keerti Parthasarathy <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: keertip

pkg/analysis_server/analyzer_use_new_elements.txt

@@ -134,7 +134,6 @@ lib/src/lsp/handlers/handler_cancel_request.dart
 lib/src/lsp/handlers/handler_change_workspace_folders.dart
 lib/src/lsp/handlers/handler_code_actions.dart
 lib/src/lsp/handlers/handler_code_lens.dart
-lib/src/lsp/handlers/handler_completion.dart
 lib/src/lsp/handlers/handler_dart_text_document_content_provider.dart
 lib/src/lsp/handlers/handler_document_color.dart
 lib/src/lsp/handlers/handler_document_highlights.dart

pkg/analysis_server/lib/plugin/protocol/protocol_dart.dart

@@ -19,7 +19,7 @@ Element convertElement(engine.Element element) {
   var name = getElementDisplayName(element);
   var elementTypeParameters = _getTypeParametersString(element);
   var aliasedType = getAliasedTypeString(element);
-  var elementParameters = _getParametersString(element);
+  var elementParameters = getParametersString(element);
   var elementReturnType = getReturnTypeString(element);
   return Element(
     kind,
@@ -188,7 +188,7 @@ String getElementDisplayName2(engine.Element2 element) {
   }
 }
 
-String? _getParametersString(engine.Element element) {
+String? getParametersString(engine.Element element) {
   // TODO(scheglov): expose the corresponding feature from ExecutableElement
   List<engine.ParameterElement> parameters;
   if (element is engine.ExecutableElement) {