unittest step2: bye bye to multiple entrypoints for unittest

client/tests/client/dom/PerformanceApiTest.dart

 #library('PerformanceApiTest');
-#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('PerformanceApi', () {
     // Check that code below will not throw exceptions.
     var requestStart = window.performance.timing.requestStart;
     var responseStart = window.performance.timing.responseStart;
     var responseEnd = window.performance.timing.responseEnd;
     
     var loading = window.performance.timing.domLoading;
     var loadedStart = window.performance.timing.domContentLoadedEventStart;
     var loadedEnd = window.performance.timing.domContentLoadedEventEnd;
     var complete = window.performance.timing.domComplete;
 
     var loadEventStart = window.performance.timing.loadEventStart;
   });
 }