Don't resynthesize invalid constant expressions at all.

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 260 matching lines @@
 
   void writeExpression(AstNode e, [Expression enclosing]) {
     bool needsParenthesis = e is Expression &&
         enclosing != null &&
         e.precedence < enclosing.precedence;
 
     if (needsParenthesis) {
       buffer.write('(');
     }
 
-    if (e is Annotation) {
+    if (e == null) {
+      buffer.write('<null>');
+    } else if (e is Annotation) {
       buffer.write('@');
       writeExpression(e.name);
       if (e.constructorName != null) {
         buffer.write('.');
         writeExpression(e.constructorName);
       }
       if (e.arguments != null) {
         writeList('(', ')', e.arguments.arguments, ', ', writeExpression,
             includeEmpty: true);
       }
@@ skipping 597 matching lines @@
     return components.join(';');
   }
 }
 
 class _Replacement {
   final int offset;
   final int end;
   final String text;
   _Replacement(this.offset, this.end, this.text);
 }