unittest step2: bye bye to multiple entrypoints for unittest

client/tests/client/dom/XHRTest.dart

 #library('XHRTest');
-#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);
   asyncTest('XHR', 1, () {
     XMLHttpRequest xhr = new XMLHttpRequest();
     // TODO: figure out how to place a resource file with fixed name alongside
     // the .html file.
     xhr.open('GET', 'MISSING-FILE', true);
     xhr.addEventListener('readystatechange', (event) {
         if (xhr.readyState == 4) {
           Expect.equals(0, xhr.status);
           Expect.stringEquals('', xhr.responseText);
           callbackDone();
         }
       }, true);
     xhr.send();
   });
 }