Fix issue 8218: "part of" directive comes after dartdoc comment.

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.
 
+part of unittest;
+
 /**
  * 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. */
-
-part of unittest;
-
 class TestCase {
   /** Identifier for this test. */
   final int id;
 
   /** A description of what the test is specifying. */
   final String description;
 
   /** The setup function to call before the test, if any. */
   Function _setUp;
 
@@ skipping 96 matching lines @@
     }
   }
 
   void error(String messageText, [String stack = '']) {
     result = ERROR;
     message = messageText;
     stackTrace = stack;
     _complete();
   }
 }