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 {
+}
+
+class Adder {
+  final x;
+  final y;
+  Adder(x, y) : this.x = x, this.y = y;
+  add() => x + y;
+}
+
+test(expected, x) {
+  Expect.equals(expected, x.toString());
+  Expect.equals(expected, new Adder("", x).add());
+}
+
+main() {
+  test("Instance of 'Object'", new Object());
+  test("Instance of 'A'", new A());
+  test("Instance of 'List'", []);
+  test("1", 1);
+}