When a closure is extracted as a method, it does not have any return type.

pkg/analysis_server/lib/src/services/refactoring/extract_method.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_method;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol_server.dart' hide Element;
 import 'package:analysis_server/src/services/correction/name_suggestion.dart';
@@ skipping 597 matching lines @@
       result.addFatalError(format(
           'Ambiguous return value: Selected block contains more than one '
           'assignment to local variables. Affected variables are:\n\n{0}',
           sb.toString().trim()));
     }
     // done
     return result;
   }
 
   void _initializeReturnType() {
-    if (_returnType == null) {
+    if (_selectionFunctionExpression != null) {
+      returnType = '';
+    } else if (_returnType == null) {
       returnType = 'void';
     } else {
       returnType = _getTypeCode(_returnType);
     }
     if (returnType == 'dynamic') {
       returnType = '';
     }
   }
 
   /**
@@ skipping 513 matching lines @@
       return false;
     }
     for (int i = 0; i < parameterTypes.length; i++) {
       if (other.parameterTypes[i] != parameterTypes[i]) {
         return false;
       }
     }
     return true;
   }
 }