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";
+
+void main() {
+  Map m = new Map();
+  
+  m[0] = 0;
+  m[1] = 1;
+  m[2] = m;
+  
+  Expect.equals('{0: 0, 1: 1, 2: {...}}', m.toString());
+  
+  m[1] = new ThrowOnToString();
+  Expect.throws(m.toString, (e) => e == "Bad!");
+  m[1] = 1;
+  Expect.equals('{0: 0, 1: 1, 2: {...}}', m.toString());
+}
+
+class ThrowOnToString {
+  
+  String toString() {
+    throw "Bad!";
+  }
+}