Provide better ToCString description for closures (issue 2680).

tests/language/src/LocalFunction3Test.dart

Index: tests/language/src/LocalFunction3Test.dart
===================================================================
--- tests/language/src/LocalFunction3Test.dart	(revision 6857)
+++ tests/language/src/LocalFunction3Test.dart	(working copy)
@@ -22,7 +22,8 @@
       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() {