Record names of constructors and use that to implement Object.toString.

dart/frog/tests/leg_only/src/ToStringTest.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.
+
+class A extends Object {
+}
+
+main() {
+  Expect.equals("Instance of 'Object'", new Object().toString());
+  Expect.equals("Instance of 'Object'", "" + new Object());
+  Expect.equals("Instance of 'A'", new A().toString());
+  Expect.equals("Instance of 'A'", "" + new A());
+  Expect.equals("Instance of 'List'", [].toString());
+  Expect.equals("Instance of 'List'", "" + []);
+  Expect.equals("1", "" + 1);
+  Expect.equals("1", (1).toString());
+}