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

pkg/unittest/lib/src/map_matchers.dart

 // Copyright (c) 2012, 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 matcher;
+
 /**
  * Returns a matcher which matches maps containing the given [value].
  */
-
-part of matcher;
-
 Matcher containsValue(value) => new _ContainsValue(value);
 
 class _ContainsValue extends BaseMatcher {
   final _value;
 
   const _ContainsValue(this._value);
 
   bool matches(item, MatchState matchState) => item.containsValue(_value);
   Description describe(Description description) =>
       description.add('contains value ').addDescriptionOf(_value);
@@ skipping 28 matching lines @@
           add(" doesn't contain key ").addDescriptionOf(_key);
     } else {
       mismatchDescription.add(' contains key ').addDescriptionOf(_key).
           add(' but with value ');
       _valueMatcher.describeMismatch(item[_key], mismatchDescription,
           matchState, verbose);
       return mismatchDescription;
     }
   }
 }