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

pkg/analysis_server/lib/src/generated_protocol.dart

Index: pkg/analysis_server/lib/src/generated_protocol.dart
diff --git a/pkg/analysis_server/lib/src/generated_protocol.dart b/pkg/analysis_server/lib/src/generated_protocol.dart
index 62b3ce88d8f751f50fcbea7593e7b948ff973110..844635f8f86ebdff8a1c235bb9eb828bc64bdfaf 100644
--- a/pkg/analysis_server/lib/src/generated_protocol.dart
+++ b/pkg/analysis_server/lib/src/generated_protocol.dart
@@ -10538,7 +10538,8 @@ class ExtractMethodFeedback extends RefactoringFeedback implements HasToJson {
   int length;
 
   /**
-   * The proposed return type for the method.
+   * The proposed return type for the method. If the returned element does not
+   * have a declared return type, this field will contain an empty string.
    */
   String returnType;