unittest step2: bye bye to multiple entrypoints for unittest

client/tests/client/dom/ExceptionsTest.dart

 #library('ExceptionsTest');
-#import('../../../../lib/unittest/unittest_dom.dart');
+#import('../../../../lib/unittest/unittest.dart');
+#import('../../../../lib/unittest/dom_config.dart');
 #import('dart:dom');
 
 main() {
-  forLayoutTests();
+  useDomConfiguration(isLayoutTest: true);
   test('DOMException', () {
     try {
       window.webkitNotifications.createNotification('', '', '');
     } catch (DOMException e) {
       Expect.equals(DOMException.SECURITY_ERR, e.code);
       Expect.equals('SECURITY_ERR', e.name);
       Expect.equals('SECURITY_ERR: DOM Exception 18', e.message);
     }
   });
   test('EventException', () {
     final event = window.document.createEvent('Event');
     // Intentionally do not initialize it!
     try {
       window.document.dispatchEvent(event);
     } catch (EventException e) {
       Expect.equals(EventException.UNSPECIFIED_EVENT_TYPE_ERR, e.code);
       Expect.equals('UNSPECIFIED_EVENT_TYPE_ERR', e.name);
       Expect.equals('UNSPECIFIED_EVENT_TYPE_ERR: DOM Events Exception 0', e.message);
     }
   });
 }