Added an example of the use of predicate.

pkg/unittest/lib/src/core_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 that matches empty strings, maps or collections.
  */
 const Matcher isEmpty = const _Empty();
@@ skipping 614 matching lines @@
     }
     return false;
   }
 
   Description describe(Description description) =>
       description.add('is in ').addDescriptionOf(_expected);
 }
 
 /**
  * Returns a matcher that uses an arbitrary function that returns
- * true or false for the actual value.
+ * true or false for the actual value. For example:
+ *
+ *     expect(v, predicate((x) => ((x % 2) == 0), "is even"))
  */
-Matcher predicate(f, [description ='satisfies function']) =>
+Matcher predicate(Function f, [description ='satisfies function']) =>
     new _Predicate(f, description);
 
 class _Predicate extends BaseMatcher {
 
-  final _matcher;
+  final Function _matcher;
   final String _description;
 
   const _Predicate(this._matcher, this._description);
 
   bool matches(item, MatchState matchState) => _matcher(item);
 
   Description describe(Description description) =>
       description.add(_description);
 }
 
@@ skipping 40 matching lines @@
       description.add(_featureDescription).add(' ').addDescriptionOf(_matcher);
 
   Description describeMismatch(item, Description mismatchDescription,
                                MatchState matchState, bool verbose) {
     mismatchDescription.add(_featureName).add(' ');
     _matcher.describeMismatch(matchState.state['feature'], mismatchDescription,
         matchState.state['innerState'], verbose);
     return mismatchDescription;
   }
 }