unittest step2: bye bye to multiple entrypoints for unittest

client/tests/client/dom/CrossFrameTest.dart

 #library('CrossFrameTest');
-#import('../../../../lib/unittest/unittest_dom.dart');
+#import('../../../../lib/unittest/unittest.dart');
+#import('../../../../lib/unittest/dom_config.dart');
 #import('dart:dom');
 
 main() {
-  forLayoutTests();
+  useDomConfiguration();
 
   test('contentWindow', () {
       final iframe = document.createElement('iframe');
       document.body.appendChild(iframe);
       final frameWindow = iframe.contentWindow;
 
       // Test this field to ensure a valid Dart wrapper.
       Expect.isNull(frameWindow.dartObjectLocalStorage);
 
       // Ensure that the frame's document is inaccessible via window.
       Expect.throws(() => frameWindow.document);
     });
 
   test('contentDocument', () {
       final iframe = document.createElement('iframe');
       document.body.appendChild(iframe);
 
       // Ensure that the frame's document is inaccessible.
       Expect.throws(() => iframe.contentDocument);
     });
 }