Use a better way to compute InterfaceTypes least upper bound.

pkg/analysis_server/lib/src/services/refactoring/extract_method.dart

Index: pkg/analysis_server/lib/src/services/refactoring/extract_method.dart
diff --git a/pkg/analysis_server/lib/src/services/refactoring/extract_method.dart b/pkg/analysis_server/lib/src/services/refactoring/extract_method.dart
index 2673fef385dca2c2248a3b7076982a7fa7bcb434..40cad52155906b859cf90c9a53a674fd6d05c1cf 100644
--- a/pkg/analysis_server/lib/src/services/refactoring/extract_method.dart
+++ b/pkg/analysis_server/lib/src/services/refactoring/extract_method.dart
@@ -1113,7 +1113,11 @@ class _ReturnTypeComputer extends RecursiveAstVisitor {
       if (returnType == null) {
         returnType = type;
       } else {
-        returnType = returnType.getLeastUpperBound(type);
+        if (returnType is InterfaceType && type is InterfaceType) {
+          returnType = InterfaceType.getSmartLeastUpperBound(returnType, type);
+        } else {
+          returnType = returnType.getLeastUpperBound(type);
+        }
       }
     }
   }