Any field formal parameter has DefaultFieldFormalParameterElementImpl.

pkg/analyzer/test/src/summary/resynthesize_test.dart

 // Copyright (c) 2015, 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.src.serialization.elements_test;
 
 import 'dart:convert';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 1103 matching lines @@
   }
 
   test_class_constructor_field_formal_untyped_typed() {
     checkLibrary('class C { var x; C(int this.x); }');
   }
 
   test_class_constructor_field_formal_untyped_untyped() {
     checkLibrary('class C { var x; C(this.x); }');
   }
 
+  test_class_constructor_fieldFormal_named_noDefault() {
+    checkLibrary('class C { int x; C({this.x}); }');
+  }
+
+  test_class_constructor_fieldFormal_named_withDefault() {
+    checkLibrary('class C { int x; C({this.x: 42}); }');
+  }
+
+  test_class_constructor_fieldFormal_optional_noDefault() {
+    checkLibrary('class C { int x; C([this.x]); }');
+  }
+
+  test_class_constructor_fieldFormal_optional_withDefault() {
+    checkLibrary('class C { int x; C([this.x = 42]); }');
+  }
+
   test_class_constructor_implicit() {
     checkLibrary('class C {}');
   }
 
   test_class_constructor_implicit_type_params() {
     checkLibrary('class C<T, U> {}');
   }
 
   test_class_constructor_params() {
     checkLibrary('class C { C(x, y); }');
@@ skipping 2347 matching lines @@
       fail('Unexpectedly tried to get unlinked summary for $uri');
     }
     return serializedUnit;
   }
 
   @override
   bool hasLibrarySummary(String uri) {
     return true;
   }
 }