Resynthesize field formal parameters from Kernel.

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

 // Copyright (c) 2017, 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.
 
 import 'dart:io';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
@@ skipping 582 matching lines @@
       buffer.write(' show ');
       buffer.write(e.shownNames.join(', '));
     } else if (e is HideElementCombinator) {
       buffer.write(' hide ');
       buffer.write(e.hiddenNames.join(', '));
     }
   }
 
   void writeParameterElement(ParameterElement e) {
     String defaultValueSeparator;
-    Expression defaultValue = e is ConstVariableElement
-        ? (e as ConstVariableElement).constantInitializer
-        : null;
+    Expression defaultValue;
     String closeString;
     ParameterKind kind = e.parameterKind;
     if (kind == ParameterKind.REQUIRED) {
       closeString = '';
     } else if (kind == ParameterKind.POSITIONAL) {
       buffer.write('[');
       defaultValueSeparator = ' = ';
+      defaultValue = (e as ConstVariableElement).constantInitializer;
       closeString = ']';
     } else if (kind == ParameterKind.NAMED) {
       buffer.write('{');
       defaultValueSeparator = ': ';
+      defaultValue = (e as ConstVariableElement).constantInitializer;
       closeString = '}';
     } else {
       fail('Unknown parameter kind: $kind');
     }
 
+    // Kernel desugars omitted default parameter values to 'null'.
+    // Analyzer does not set initializer at all.
+    // It is not an interesting distinction, so we skip NullLiteral(s).
+    if (defaultValue is NullLiteral) {
+      defaultValue = null;
+    }
+
     writeMetadata(e, '', ' ');
 
     writeIf(e.isCovariant, 'covariant ');
     writeIf(e.isFinal, 'final ');
 
     writeType2(e.type);
 
     if (e is FieldFormalParameterElement) {
       buffer.write('this.');
     }
@@ skipping 239 matching lines @@
     return components.join(';');
   }
 }
 
 class _Replacement {
   final int offset;
   final int end;
   final String text;
   _Replacement(this.offset, this.end, this.text);
 }