Added a new Matcher, FeatureMatcher, that is a useful abstract base class

pkg/unittest/test/matchers_test.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.
 
 #library('matcherTest');
 #import('../../../pkg/unittest/unittest.dart');
 #source('test_utils.dart');
 
 doesNotThrow() {}
 doesThrow() { throw 'X'; }
 
 class PrefixMatcher extends BaseMatcher {
   final String _prefix;
   const PrefixMatcher(this._prefix);
   bool matches(item, MatchState matchState) {
     return item is String &&
         (collapseWhitespace(item)).startsWith(collapseWhitespace(_prefix));
   }
 
   Description describe(Description description) =>
     description.add('a string starting with ').
         addDescriptionOf(collapseWhitespace(_prefix)).
         add(' ignoring whitespace');
 }
 
+class Widget {
+  int price;
+}
+
+class HasPrice extends FeatureMatcher {
+  const HasPrice(matcher) :
+    super("Widget with a price that is", "price", matcher);
+  featureValueOf(actual) => actual.price;
+}
+
 void main() {
 
   initUtils();
 
   // Core matchers
 
   group('Core matchers', () {
 
     test('isTrue', () {
       shouldPass(true, isTrue);
@@ skipping 481 matching lines @@
 
   group('Predicate Matchers', () {
     test('isInstanceOf', () {
       shouldFail(0, predicate((x) => x is String,
                               description: "an instance of String"),
           "Expected: an instance of String but: was <0>.");
       shouldPass('cow', predicate((x) => x is String,
                                   description: "an instance of String"));
     });
   });
+
+  group('Feature Matchers', () {
+    test("Feature Matcher", () {
+      var w = new Widget();
+      w.price = 10;
+      shouldPass(w, new HasPrice(greaterThan(0)));
+      shouldFail(w, new HasPrice(greaterThan(10)),
+          'Expected: Widget with a price that is a value greater than <10> '
+          'but: price was <10>.');
+    });
+  });
 }