dart2dart: fix top level field declarations.

tests/compiler/dart2js/unparser_test.dart

Index: tests/compiler/dart2js/unparser_test.dart
diff --git a/tests/compiler/dart2js/unparser_test.dart b/tests/compiler/dart2js/unparser_test.dart
index a810b34caa79d267c299f7eb65ded1f4d077f234..bf400f8aaeb5db1f9aa1c94b5407776df243bdb3 100644
--- a/tests/compiler/dart2js/unparser_test.dart
+++ b/tests/compiler/dart2js/unparser_test.dart
@@ -111,6 +111,11 @@ main() {
   });
 }
 
+testTopLevelField() {
+  final src = 'final String x="asd";main(){x;}';
+  testDart2Dart(src, (String s) => Expect.equals(src, s));
+}
+
 main() {
   testGenericTypes();
   testForLoop();
@@ -120,4 +125,5 @@ main() {
   testNativeMethods();
   testPrefixIncrements();
   testSimpleFileUnparse();
+  testTopLevelField();
 }