Issue 25253. Improve collecting names that may conflict with the extracted local variable.

pkg/analysis_server/lib/src/services/refactoring/extract_local.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.src.refactoring.extract_local;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analysis_server/src/protocol_server.dart' hide Element;
@@ skipping 58 matching lines @@
     if (_isPartOfConstantExpression(rootExpression)) {
       return "const";
     } else {
       return "var";
     }
   }
 
   @override
   Future<RefactoringStatus> checkFinalConditions() {
     RefactoringStatus result = new RefactoringStatus();
-    if (excludedVariableNames.contains(name)) {
-      result.addWarning(format(
-          "A variable with name '{0}' is already defined in the visible scope.",
-          name));
-    }
+    result.addStatus(checkName());
     return new Future.value(result);
   }
 
   @override
   Future<RefactoringStatus> checkInitialConditions() {
     RefactoringStatus result = new RefactoringStatus();
     // selection
     result.addStatus(_checkSelection());
     if (result.hasFatalError) {
       return new Future.value(result);
     }
     // occurrences
     _prepareOccurrences();
     _prepareOffsetsLengths();
     // names
     excludedVariableNames =
         utils.findPossibleLocalVariableConflicts(selectionOffset);
     _prepareNames();
     // done
     return new Future.value(result);
   }
 
   @override
   RefactoringStatus checkName() {
-    return validateVariableName(name);
+    RefactoringStatus result = new RefactoringStatus();
+    result.addStatus(validateVariableName(name));
+    if (excludedVariableNames.contains(name)) {
+      result.addError(
+          format("The name '{0}' is already used in the scope.", name));
+    }
+    return result;
   }
 
   @override
   Future<SourceChange> createChange() {
     SourceChange change = new SourceChange(refactoringName);
     // prepare occurrences
     List<SourceRange> occurrences;
     if (extractAll) {
       occurrences = this.occurrences;
     } else {
@@ skipping 557 matching lines @@
 
   _TokenLocalElementVisitor(this.map);
 
   visitSimpleIdentifier(SimpleIdentifier node) {
     Element element = node.staticElement;
     if (element is LocalVariableElement) {
       map[node.token] = element;
     }
   }
 }