Move unittest from lib to pkg.

tests/html/performance_api_test.dart

 #library('PerformanceApiTest');
-#import('../../lib/unittest/unittest.dart');
-#import('../../lib/unittest/html_config.dart');
+#import('../../pkg/unittest/unittest.dart');
+#import('../../pkg/unittest/html_config.dart');
 #import('dart:html');
 
 main() {
   useHtmlConfiguration();
   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;
   });
 }