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

tests/utils/recursive_import_test.dart

 // Copyright (c) 2012, 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.
 
 // Test of "recursive" imports using the dart2js compiler API.
 
 import '../../sdk/lib/_internal/compiler/compiler.dart';
 import 'dart:uri';
 
 const CORE_LIB = """
@@ skipping 34 matching lines @@
       // Simulate an OS error.
       throw 'Path length exceeded';
     } else if (uri.scheme == "main") {
       count++;
       source = RECURSIVE_MAIN;
     } else if (uri.scheme == "lib") {
       if (uri.path.endsWith("/core.dart")) {
         source = CORE_LIB;
       } else if (uri.path.endsWith('_patch.dart')) {
         source = '';
+      } else if (uri.path.endsWith('isolate_helper.dart')) {
+        source = 'class _WorkerStub {}';
       } else {
         source = "library lib${uri.path.replaceAll('/', '.')};";
       }
     } else {
      throw "unexpected URI $uri";
     }
     completer.complete(source);
     return completer.future;
   }
 
@@ skipping 17 matching lines @@
                         new Uri.fromComponents(scheme: 'lib', path: '/'),
                         new Uri.fromComponents(scheme: 'package', path: '/'),
                         provider, handler).value;
   Expect.isNull(code);
   Expect.isTrue(10 < count);
   // Two warnings for each time RECURSIVE_MAIN is read, except the
   // first time.
   Expect.equals(2 * (count - 1), warningCount);
   Expect.equals(1, errorCount);
 }