Handle top level fields with Kernel.

tests/compiler/dart2js/kernel/literals_test.dart

 // Copyright (c) 2016, 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 'package:test/test.dart';
 
 import 'helper.dart' show check;
 
 main() {
   test('compile function that returns a literal list', () {
     return check('main() { return [1, 2, 3]; }');
   });
   test('compile function that returns a const list', () {
     return check('main() { return const [1, 2, 3]; }');
   });
   test('compile function that returns a literal map', () {
     return check('main() { return {"a": 1, "b": 2, "c": 3}; }');
   });
   test('compile function that returns a const map', () {
     return check('main() { return const {"a": 1, "b": 2, "c": 3}; }');
   });
+  test('compile top level string field ', () {
+    return check('String foo = (() { return "a";})(); main() { return foo; }');
+  });
 }