Any field formal parameter has DefaultFieldFormalParameterElementImpl.

pkg/analyzer/lib/src/summary/resynthesize.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 summary_resynthesizer;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 1461 matching lines @@
     return element;
   }
 
   /**
    * Resynthesize a [ParameterElement].
    */
   ParameterElement buildParameter(UnlinkedParam serializedParameter) {
     ParameterElementImpl parameterElement;
     if (serializedParameter.isInitializingFormal) {
       FieldFormalParameterElementImpl initializingParameter;
-      if (serializedParameter.defaultValue != null) {
+      if (serializedParameter.kind == UnlinkedParamKind.required) {
+        initializingParameter = new FieldFormalParameterElementImpl(
+            serializedParameter.name, serializedParameter.nameOffset);
+      } else {
         DefaultFieldFormalParameterElementImpl defaultParameter =
             new DefaultFieldFormalParameterElementImpl(
                 serializedParameter.name, serializedParameter.nameOffset);
         initializingParameter = defaultParameter;
-        defaultParameter.constantInitializer =
-            _buildConstExpression(serializedParameter.defaultValue);
-      } else {
-        initializingParameter = new FieldFormalParameterElementImpl(
-            serializedParameter.name, serializedParameter.nameOffset);
+        if (serializedParameter.defaultValue != null) {
+          defaultParameter.constantInitializer =
+              _buildConstExpression(serializedParameter.defaultValue);
+        }
       }
       parameterElement = initializingParameter;
       initializingParameter.field = fields[serializedParameter.name];
     } else {
       if (serializedParameter.kind == UnlinkedParamKind.required) {
         parameterElement = new ParameterElementImpl(
             serializedParameter.name, serializedParameter.nameOffset);
       } else {
         DefaultParameterElementImpl defaultParameter =
             new DefaultParameterElementImpl(
@@ skipping 679 matching lines @@
                 }
               : () => this.element;
       // TODO(paulberry): Is it a bug that we have to pass `false` for
       // isInstantiated?
       return new DeferredFunctionTypeImpl(computer, null, typeArguments, false);
     } else {
       return null;
     }
   }
 }