Handle null literal in compile_from_dill

tests/compiler/dart2js/kernel/compile_from_dill_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.
 
 // Partial test that the closed world computed from [WorldImpact]s derived from
 // kernel is equivalent to the original computed from resolution.
 library dart2js.kernel.compile_from_dill_test;
 
 import 'dart:async';
 import 'dart:io';
@@ skipping 16 matching lines @@
 import '../equivalence/check_functions.dart';
 import '../equivalence/check_helpers.dart';
 import '../serialization/helper.dart';
 import 'test_helpers.dart';
 
 import 'compiler_helper.dart';
 
 const SOURCE = const {
   'main.dart': '''
 main() {
+  return null;

[Siggi Cherem (dart-lang), 2017/05/30 22:45:55]

it's not empty! it's not empty! :)

 }
 '''
 };
 
 main(List<String> args) {
   asyncTest(() async {
     await mainInternal(args);
   });
 }
 
@@ skipping 99 matching lines @@
     }
     Map<String, BufferedOutputSink> map2 = collector2.outputMap[outputType];
     checkSets(map1.keys, map2.keys, 'output', equality);
     map1.forEach((String name, BufferedOutputSink output1) {
       BufferedOutputSink output2 = map2[name];
       Expect.stringEquals(output1.text, output2.text);
     });
   });
   return ResultKind.success;
 }