Fix new isolate_unhandled_exception tests to make them compatible for dartium testing.

tests/vm/dart/isolate_unhandled_exception_uri_helper.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.
 
+library isolate_unhandled_exception_uri_helper;
+
 import 'dart:isolate';
 
 // Isolate script that throws an uncaught exception, which is caught by an
 // uncaught exception handler.
 
 void main() {
   port.receive((message, replyTo) {
     if (message == 'throw exception') {
       throw new RuntimeError('ignore this exception');
     }
     replyTo.call('hello');
     port.close();
   });
 }
 
 bool _unhandledExceptionCallback(IsolateUnhandledException e) {
   return e.source.message == 'ignore this exception';
 }