Move toString() to collection classes.

tests/corelib/map_to_string_test.dart

 // Copyright (c) 2013, 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.
 
 import "package:expect/expect.dart";
 
-class A native "A" {}
-
-main() {
-  var a = [new Object()];
-  Expect.isFalse(a[0] is A);
-}
+void main() {
+  Map m = new Map();
+  
+  m[0] = 0;
+  m[1] = 1;
+  m[2] = m;
+  
+  Expect.equals('{0: 0, 1: 1, 2: {...}}', m.toString());
+}