Support locals in compile_from_dill_test.

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 22 matching lines @@
 
 const SOURCE = const {
   'main.dart': '''
 foo() => 1;
 bar() => true;
 main() {
   foo();
   bar();
   [];
   {};
-  return null;
+  var x = null;
+  return x;
 }
 '''
 };
 
 main(List<String> args) {
   asyncTest(() async {
     await mainInternal(args);
   });
 }
 
@@ skipping 110 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;
 }