Fix strong mode warning in the 'getter to method refactoring.'

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

Index: pkg/analysis_server/lib/src/services/refactoring/convert_getter_to_method.dart
diff --git a/pkg/analysis_server/lib/src/services/refactoring/convert_getter_to_method.dart b/pkg/analysis_server/lib/src/services/refactoring/convert_getter_to_method.dart
index de6e76187c23ddde62618925dd32c92c7f3c9db7..4d9d93f8d70e0e1d3242eb0640c88a3e32c0dd39 100644
--- a/pkg/analysis_server/lib/src/services/refactoring/convert_getter_to_method.dart
+++ b/pkg/analysis_server/lib/src/services/refactoring/convert_getter_to_method.dart
@@ -58,11 +58,13 @@ class ConvertGetterToMethodRefactoringImpl extends RefactoringImpl
       FieldElement field = element.variable;
       Set<ClassMemberElement> elements =
           await getHierarchyMembers(searchEngine, field);
-      await Future.forEach(elements, (FieldElement field) {
-        PropertyAccessorElement getter = field.getter;
-        if (!getter.isSynthetic) {
-          _updateElementDeclaration(getter);
-          return _updateElementReferences(getter);
+      await Future.forEach(elements, (ClassMemberElement member) {
+        if (member is FieldElement) {
+          PropertyAccessorElement getter = member.getter;
+          if (!getter.isSynthetic) {
+            _updateElementDeclaration(getter);
+            return _updateElementReferences(getter);
+          }
         }
       });
     }