corelib: Do not print value in ArgumentError.notNull

tests/corelib/errors_test.dart

 // Copyright (c) 2015, 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";
 
 // Test that error constructors do what they are documented as doing.
 
 main() {
   Expect.equals("Invalid argument(s)",
                 new ArgumentError().toString());
   Expect.equals("Invalid argument(s): message",
                 new ArgumentError("message").toString());
   Expect.equals("Invalid argument: null",
                 new ArgumentError.value(null).toString());
   Expect.equals("Invalid argument: 42",
                 new ArgumentError.value(42).toString());
   Expect.equals("Invalid argument: \"bad\"",
                 new ArgumentError.value("bad").toString());
   Expect.equals("Invalid argument (foo): null",
                 new ArgumentError.value(null, "foo").toString());
   Expect.equals("Invalid argument (foo): 42",
                 new ArgumentError.value(42, "foo").toString());
   Expect.equals("Invalid argument (foo): message: 42",
                 new ArgumentError.value(42, "foo", "message").toString());
   Expect.equals("Invalid argument: message: 42",
                 new ArgumentError.value(42, null, "message").toString());
-  Expect.equals("Invalid argument: Must not be null: null",
+  Expect.equals("Invalid argument(s): Must not be null",
                 new ArgumentError.notNull().toString());
-  Expect.equals("Invalid argument (foo): Must not be null: null",
+  Expect.equals("Invalid argument(s) (foo): Must not be null",
                 new ArgumentError.notNull("foo").toString());
 
   Expect.equals("RangeError",
                 new RangeError(null).toString());
   Expect.equals("RangeError: message",
                 new RangeError("message").toString());
   Expect.equals("RangeError: Value not in range: 42",
                 new RangeError.value(42).toString());
   Expect.equals("RangeError (foo): Value not in range: 42",
                 new RangeError.value(42, "foo").toString());
@@ skipping 27 matching lines @@
                  new RangeError.index(42, [1, 2, 3],
                                       null, "message").toString());
   Expect.equals("RangeError (foo): message: "
                 "index should be less than 2: 42",
                  new RangeError.index(42, [1, 2, 3],
                                       "foo", "message", 2).toString());
   Expect.equals("RangeError: Index out of range: "
                 "index must not be negative: -5",
                 new RangeError.index(-5, [1, 2, 3]).toString());
 }