Initial --modules=es6 support

test/codegen/es6_modules.dart

Index: test/codegen/es6_modules.dart
diff --git a/test/codegen/es6_modules.dart b/test/codegen/es6_modules.dart
new file mode 100644
index 0000000000000000000000000000000000000000..6370fdc26d935d3c48fd592fcf62d088a6ab5d5b
--- /dev/null
+++ b/test/codegen/es6_modules.dart
@@ -0,0 +1,25 @@
+library test;
+
+import 'dart:js';
+
+typedef void Callback({int i});
+
+class A {}
+class _A {}
+class B<T> {}
+class _B<T> {}
+
+f() {}
+_f() {}
+
+const String constant = "abc";
+final String finalConstant = "abc";
+final String lazy = (() {
+  print('lazy');
+  return "abc";
+})();
+String mutable = "abc";
+String lazyMutable = (() {
+  print('lazyMutable');
+  return "abc";
+})();