Improve matcher descriptions that match against objects, and thus fix a bug

pkg/unittest/test/mock_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 mock_test;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/mock.dart';
 
 class MockList extends Mock implements List {
 }
@@ skipping 631 matching lines @@
     m1.clearLogs();
     expect(log.logs, hasLength(6));
     expect(log.logs.every((e) => e.mockName == 'm2' || e.mockName == 'm3'),
         isTrue);
     m2.clearLogs();
     expect(log.logs, hasLength(3));
     expect(log.logs.every((e) => e.mockName =='m3'), isTrue);
     m3.clearLogs();
     expect(log.logs, hasLength(0));
   });
+
+  test("Mocking: instances", () {
+    var alice = new Object();
+    var bob = new Object();
+    var m = new Mock();
+    m.when(callsTo("foo", alice)).alwaysReturn(true);
+    m.when(callsTo("foo", bob)).alwaysReturn(false);
+    expect(m.foo(alice), true);
+    expect(m.foo(bob), false);
+    expect(m.foo(alice), true);
+    expect(m.foo(bob), false);
+  });
 }