Resynthesize ?? expressions from Kernel.

pkg/analyzer/test/src/summary/resynthesize_kernel_test.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.
 
 library analyzer.test.src.summary.resynthesize_kernel_test;
 
 import 'dart:async';
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
@@ skipping 95 matching lines @@
       var library = libraryMap[testUriStr];
       print(_getLibraryText(library));
     }
 
     var resynthesizer =
         new KernelResynthesizer(context, kernelResult.types, libraryMap);
     return resynthesizer.getLibrary(testUriStr);
   }
 
   @failingTest
+  @fastaProblem
   test_class_constructor_field_formal_multiple_matching_fields() async {
     // Fasta does not generate the class.
     // main() with a fatal error is generated instead.
     await super.test_class_constructor_field_formal_multiple_matching_fields();
   }
 
   @failingTest
   @fastaProblem
   test_class_type_parameters_bound() async {
     // Fasta does not provide a flag for explicit vs. implicit Object bound.
@@ skipping 104 matching lines @@
   }
 
   @failingTest
   @fastaProblem
   test_const_reference_unresolved_prefix2() async {
     // https://github.com/dart-lang/sdk/issues/30267
     await super.test_const_reference_unresolved_prefix2();
   }
 
   @failingTest
-  test_const_topLevel_ifNull() async {
-    await super.test_const_topLevel_ifNull();
-  }
-
-  @failingTest
   @fastaProblem
   test_const_topLevel_super() async {
     // https://github.com/dart-lang/sdk/issues/30267
     await super.test_const_topLevel_super();
   }
 
   @failingTest
   @fastaProblem
   test_const_topLevel_this() async {
     // https://github.com/dart-lang/sdk/issues/30267
@@ skipping 580 matching lines @@
   @override
   Future<List<int>> readAsBytes() async {
     return file.readAsBytesSync();
   }
 
   @override
   Future<String> readAsString() async {
     return file.readAsStringSync();
   }
 }