Handle top level fields with Kernel.

tests/compiler/dart2js/kernel/literals_test.dart

Index: tests/compiler/dart2js/kernel/literals_test.dart
diff --git a/tests/compiler/dart2js/kernel/literals_test.dart b/tests/compiler/dart2js/kernel/literals_test.dart
index 4bdf2714bb1726976ae857102f660f2f3fb49eac..5cc337af2cf065de9eab8eaac0331bbb9fff150d 100644
--- a/tests/compiler/dart2js/kernel/literals_test.dart
+++ b/tests/compiler/dart2js/kernel/literals_test.dart
@@ -19,4 +19,7 @@ main() {
   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; }');
+  });
 }