Restructure pkg/unittest and pkg/webdriver to follow the pub conventions.

pkg/unittest/lib/src/test_case.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * testcase.dart: this file is sourced by unittest.dart. It defines [TestCase]
  * and assumes unittest defines the type [TestFunction].
  */
 
 /** Summarizes information about a single test case. */
@@ skipping 87 matching lines @@
       _doneTeardown = true;
     }
     _config.onTestResult(this);
   }
 
   void pass() {
     result = PASS;
     _complete();
   }
 
-  void fail(String messageText, String stack) {
+  void fail(String messageText, [String stack = '']) {
     if (result != null) {
       if (result == PASS) {
         error('Test failed after initially passing: $messageText', stack);
       } else if (result == FAIL) {
         error('Test failed more than once: $messageText', stack);
       }
     } else {
       result = FAIL;
       message = messageText;
       stackTrace = stack;
       _complete();
     }
   }
 
-  void error(String messageText, String stack) {
+  void error(String messageText, [String stack = '']) {
     result = ERROR;
     message = messageText;
     stackTrace = stack;
     _complete();
   }
 }