Completions with exact prefix should be prioritized - fixes #23099

pkg/analysis_server/lib/src/services/completion/local_reference_contributor.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library services.completion.contributor.dart.local;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol.dart' as protocol
     show Element, ElementKind;
@@ skipping 333 matching lines @@
   }
 }
 
 /**
  * A visitor for collecting suggestions from the most specific child [AstNode]
  * that contains the completion offset to the [CompilationUnit].
  */
 class _LocalVisitor extends LocalDeclarationVisitor {
   final DartCompletionRequest request;
   final OpType optype;
+  int privateMemberRelevance = DART_RELEVANCE_DEFAULT;
 
   _LocalVisitor(this.request, int offset, this.optype) : super(offset) {
     includeLocalInheritedTypes = !optype.inStaticMethodBody;
+    if (request.replacementLength > 0) {
+      var contents = request.source.contents;
+      if (contents != null &&
+          contents.data != null &&
+          contents.data.startsWith('_', request.replacementOffset)) {
+        // If user typed identifier starting with '_'
+        // then do not suppress the relevance of private members
+        privateMemberRelevance = null;
+      }
+    }
   }
 
   @override
   void declaredClass(ClassDeclaration declaration) {
     if (optype.includeTypeNameSuggestions) {
       _addSuggestion(
           declaration.name, NO_RETURN_TYPE, protocol.ElementKind.CLASS,
           isAbstract: declaration.isAbstract,
           isDeprecated: isDeprecated(declaration));
     }
@@ skipping 180 matching lines @@
       SimpleIdentifier id, TypeName typeName, protocol.ElementKind elemKind,
       {bool isAbstract: false,
       bool isDeprecated: false,
       ClassDeclaration classDecl,
       FormalParameterList param,
       int relevance: DART_RELEVANCE_DEFAULT}) {
     CompletionSuggestion suggestion = createSuggestion(
         id, isDeprecated, relevance, typeName,
         classDecl: classDecl);
     if (suggestion != null) {
+      if (privateMemberRelevance != null &&
+          suggestion.completion.startsWith('_')) {
+        suggestion.relevance = privateMemberRelevance;
+      }
       request.addSuggestion(suggestion);
       suggestion.element = createElement(request.source, elemKind, id,
           isAbstract: isAbstract,
           isDeprecated: isDeprecated,
           parameters: param != null ? param.toSource() : null,
           returnType: typeName);
       if ((elemKind == protocol.ElementKind.METHOD ||
               elemKind == protocol.ElementKind.FUNCTION) &&
           param != null) {
         _addParameterInfo(suggestion, param);
       }
     }
   }
 
   bool _isVoid(TypeName returnType) {
     if (returnType != null) {
       Identifier id = returnType.name;
       if (id != null && id.name == 'void') {
         return true;
       }
     }
     return false;
   }
 }