Provide better ToCString description for closures (issue 2680).

tests/language/src/LocalFunctionTest.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 LocalFunctionTest {
   LocalFunctionTest() : field1 = 100, field2_ = 200  { }
   static int f(int n) {
     int a = 0;
     g(int m) {
@@ skipping 120 matching lines @@
     (LocalFunctionTest.set_field5_func)(x);
     Expect.equals(x * x, LocalFunctionTest.field5);
     return true;
   }
 
   static testExceptions() {
     var f = (int n) => n + 1;
     Expect.equals(2, f(1));
     Expect.equals(true, f is Function);
     Expect.equals(true, f is Object);
-    Expect.equals("Closure", f.toString());
+    Expect.equals(true, f.toString().startsWith("Closure"));
     bool exception_caught = false;
     try {
       f(1, 2);
     } catch (ClosureArgumentMismatchException e) {
       exception_caught = true;
     }
     Expect.equals(true, exception_caught);
     exception_caught = false;
     try {
       f();
@@ skipping 43 matching lines @@
     Expect.equals(99, doThis(10, int f(n) => n * n - 1));
     Expect.equals(99, doThis(10, (n) => n * n - 1));
     Expect.equals(99, doThis(10, f(n) => n * n - 1));
     testExceptions();
   }
 }
 
 main() {
   LocalFunctionTest.testMain();
 }