Provide better ToCString description for closures (issue 2680).

tests/language/src/LocalFunction3Test.dart

 // Copyright (c) 2011, 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.
 // Dart test program testing closures.
 
 class LocalFunction3Test {
   static testExceptions() {
     var f = (int n) { return n + 1; };
     Expect.equals(true, f is Object);
     bool exception_caught = false;
     try {
       f.xyz(0);
     } catch (NoSuchMethodException e) {
       exception_caught = true;
     }
     Expect.equals(true, exception_caught);
     exception_caught = false;
     String f_string;
     try {
       f_string = f.toString();
     } catch (NoSuchMethodException e) {
       exception_caught = true;
     }
     Expect.equals(false, exception_caught);
-    Expect.equals("Closure", f_string);
+    Expect.equals("", f_string);
+    Expect.equals(true, f_string.startsWith("Closure"));
   }
 
   static testMain() {
     testExceptions();
   }
 }
 
 main() {
   LocalFunction3Test.testMain();
 }