corelib: Do not print value in ArgumentError.notNull

tests/corelib/errors_test.dart

Index: tests/corelib/errors_test.dart
diff --git a/tests/corelib/errors_test.dart b/tests/corelib/errors_test.dart
index edfa7611e7907e0b68390cd0177f5d5dc3ea0963..c980d441f3865b786422b9ea427e9593d5bba9e5 100644
--- a/tests/corelib/errors_test.dart
+++ b/tests/corelib/errors_test.dart
@@ -25,9 +25,9 @@ main() {
                 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",