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

pkg/analysis_server/test/services/refactoring/extract_method_test.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 test.services.refactoring.extract_method;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol.dart';
 import 'package:analysis_server/src/services/correction/status.dart';
@@ skipping 1040 matching lines @@
 // end
   print(v);
 }
 ''');
     _createRefactoringForStartEndComments();
     // do check
     await refactoring.checkInitialConditions();
     expect(refactoring.returnType, 'double');
   }
 
+  test_returnType_closure() async {
+    indexTestUnit('''
+process(f(x)) {}
+main() {
+  process((x) => x * 2);
+}
+''');
+    _createRefactoringForString('(x) => x * 2');
+    // do check
+    await refactoring.checkInitialConditions();
+    expect(refactoring.returnType, '');
+  }
+
   test_returnType_statements_nullMix() async {
     indexTestUnit('''
 main(bool p) {
 // start
   if (p) {
     return 42;
   }
   return null;
 // end
 }
@@ skipping 1454 matching lines @@
    * Returns a deep copy of [refactoring] parameters.
    * There was a bug masked by updating parameter instances shared between the
    * refactoring and the test.
    */
   List<RefactoringMethodParameter> _getParametersCopy() {
     return refactoring.parameters.map((p) {
       return new RefactoringMethodParameter(p.kind, p.type, p.name, id: p.id);
     }).toList();
   }
 }