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

editor/tools/plugins/com.google.dart.server/src/com/google/dart/server/generated/types/ExtractMethodFeedback.java

Index: editor/tools/plugins/com.google.dart.server/src/com/google/dart/server/generated/types/ExtractMethodFeedback.java
diff --git a/editor/tools/plugins/com.google.dart.server/src/com/google/dart/server/generated/types/ExtractMethodFeedback.java b/editor/tools/plugins/com.google.dart.server/src/com/google/dart/server/generated/types/ExtractMethodFeedback.java
index 94c1bade24c3ef2d7239220c63fc222ac37244e7..7e446a901bab16a6144e2b8e737c783873666646 100644
--- a/editor/tools/plugins/com.google.dart.server/src/com/google/dart/server/generated/types/ExtractMethodFeedback.java
+++ b/editor/tools/plugins/com.google.dart.server/src/com/google/dart/server/generated/types/ExtractMethodFeedback.java
@@ -52,7 +52,8 @@ public class ExtractMethodFeedback extends RefactoringFeedback {
   private final 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.
    */
   private final String returnType;
 
@@ -194,7 +195,8 @@ public class ExtractMethodFeedback extends RefactoringFeedback {
   }
 
   /**
-   * 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.
    */
   public String getReturnType() {
     return returnType;