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

tests/utils/dummy_compiler_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.
 
 // Smoke test of the dart2js compiler API.
 
 library dummy_compiler;
 
 import '../../sdk/lib/_internal/compiler/compiler.dart';
 import 'dart:uri';
@@ skipping 31 matching lines @@
                   class JSString {}
                   class JSFunction {}
                   class JSInt {}
                   class JSDouble {}
                   class JSNumber {}
                   class JSNull {}
                   class JSBool {}
                   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;";
     }
   } else {
    throw "unexpected URI $uri";
   }
   completer.complete(source);
   return completer.future;
 }
 
 void handler(Uri uri, int begin, int end, String message, Diagnostic kind) {
   if (uri == null) {
     print('$kind: $message');
   } else {
     print('$uri:$begin:$end: $kind: $message');
   }
 }
 
 main() {
   String code = compile(new Uri.fromComponents(scheme: 'main'),
                         new Uri.fromComponents(scheme: 'lib', path: '/'),
                         new Uri.fromComponents(scheme: 'package', path: '/'),
                         provider, handler).value;
   if (code == null) {
     throw 'Compilation failed';
   }
 }