Update unittest to new package layout.

tests/html/isolates_test.dart

 #library('IsolatesTest');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
+#import('../../pkg/unittest/lib/unittest.dart');
+#import('../../pkg/unittest/lib/html_config.dart');
 #import('dart:html');
 #import('dart:json');
 #import('dart:isolate', prefix:'isolate');
 
 String responseFor(message) => 'response for $message';
 
 void isolateEntry() {
   bool wasThrown = false;
   try {
     window.alert('Test');
@@ skipping 29 matching lines @@
         port.call(msg2).then((response) {
           guardAsync(() {
             Expect.equals(responseFor(msg2), response);
             callback();
           });
         });
       });
     });
   });
 }