Make unit testing of the compiler work with the new isolate helper library.

tests/utils/dummy_compiler_test.dart

Index: tests/utils/dummy_compiler_test.dart
===================================================================
--- tests/utils/dummy_compiler_test.dart	(revision 16156)
+++ tests/utils/dummy_compiler_test.dart	(working copy)
@@ -49,6 +49,8 @@
                   var getInterceptor;""";
     } else if (uri.path.endsWith('js_helper.dart')) {
       source = 'library jshelper; class JSInvocationMirror {}';
+    } else if (uri.path.endsWith('isolate_helper.dart')) {
+      source = 'library isolatehelper; class _WorkerStub {}';
     } else {
       source = "library lib;";
     }