Address a number of issues related to computation of currentScript and deferred loading.

dart/tests/compiler/dart2js_native/only_pass_on_jsshell_test.dart

Index: dart/tests/compiler/dart2js_native/only_pass_on_jsshell_test.dart
diff --git a/dart/tests/lib/mirrors/unnamed_library_test.dart b/dart/tests/compiler/dart2js_native/only_pass_on_jsshell_test.dart
similarity index 66%
copy from dart/tests/lib/mirrors/unnamed_library_test.dart
copy to dart/tests/compiler/dart2js_native/only_pass_on_jsshell_test.dart
index 1e81eaab1284ae5b7f1d4973bfc114ce4572c471..8bb481142f242164cf315de7d56081a6fb28c979 100644
--- a/dart/tests/lib/mirrors/unnamed_library_test.dart
+++ b/dart/tests/compiler/dart2js_native/only_pass_on_jsshell_test.dart
@@ -2,14 +2,11 @@
 // 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.
 
-// No library declaration.
-
-import 'dart:mirrors';
-
 import 'package:expect/expect.dart';
 
-class C {}
+import 'dart:_isolate_helper';
 
 main() {
-  Expect.equals(const Symbol(""), reflectClass(C).owner.simpleName);
+  Expect.isFalse(IsolateNatives.isD8);
+  Expect.isTrue(IsolateNatives.isJsshell);
 }