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

pkg/unittest/lib/src/description.dart

Index: pkg/unittest/lib/src/description.dart
===================================================================
--- pkg/unittest/lib/src/description.dart	(revision 18358)
+++ pkg/unittest/lib/src/description.dart	(working copy)
@@ -47,6 +47,12 @@
       String description = (value == null) ? "null" : value.toString();
       if (description.startsWith('<') && description.endsWith('>')) {
           add(description);
+      } else if (description.startsWith("Instance of")) {
+        add('<');
+        add(description);
+        add(':');
+        add(value.hashCode.toString());
+        add('>');
       } else {
         add('<');
         add(description);