Any field formal parameter has DefaultFieldFormalParameterElementImpl.

pkg/analyzer/lib/src/summary/resynthesize.dart

Index: pkg/analyzer/lib/src/summary/resynthesize.dart
diff --git a/pkg/analyzer/lib/src/summary/resynthesize.dart b/pkg/analyzer/lib/src/summary/resynthesize.dart
index 53f86922a07ad0809f06fa2a877e217a899a9744..d9566ab8bccf1c82d7ffb3ce371de86e1991f534 100644
--- a/pkg/analyzer/lib/src/summary/resynthesize.dart
+++ b/pkg/analyzer/lib/src/summary/resynthesize.dart
@@ -1479,16 +1479,18 @@ class _LibraryResynthesizer {
     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];