Don't invoke user-defined code in NoSuchMethodError.toString.

dart/tests/corelib/safe_to_string_test.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.
+
+main() {
+  Expect.stringEquals('1', NoSuchMethodError.safeToString(1));
+  Expect.stringEquals('0.5', NoSuchMethodError.safeToString(0.5));
+  Expect.stringEquals('"1"', NoSuchMethodError.safeToString("1"));
+  Expect.stringEquals('"\'"', NoSuchMethodError.safeToString("'"));
+  Expect.stringEquals('"\'\'"', NoSuchMethodError.safeToString("''"));
+  Expect.stringEquals(r'"\""', NoSuchMethodError.safeToString('"'));
+  Expect.stringEquals(r'"\"\""', NoSuchMethodError.safeToString('""'));
+  Expect.stringEquals('null', NoSuchMethodError.safeToString(null));
+  Expect.stringEquals('true', NoSuchMethodError.safeToString(true));
+  Expect.stringEquals('false', NoSuchMethodError.safeToString(false));
+  Expect.stringEquals("Instance of 'Object'",
+                      NoSuchMethodError.safeToString(new Object()));
+}