Always initialize top-level `Isolate` for deferred libraries.

tests/language/deferred_only_constant_test.dart

 // Copyright (c) 2014, 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.
 
+// Testing import of only constants from a deferred library.
+
 import 'package:expect/expect.dart';
-import 'package:async_helper/async_helper.dart';
 
-import "deferred_no_such_method_lib.dart" deferred as lib;
+import "deferred_only_constant_lib.dart" deferred as lib;
 
 void main() {
-  asyncStart();
   lib.loadLibrary().then((_) {
-    Expect.equals(42, new lib.C().nonExisting());
-    asyncEnd();
+    Expect.equals(lib.constant, const ["a", "b", "c"]);
   });
 }