State that skip/take does not accept negative arguments.

tests/corelib/iterable_skip_test.dart

 // Copyright (c) 2012, 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";
 
 main() {
   List<int> list1 = <int>[1, 2, 3];
   List<int> list2 = const <int>[4, 5];
   List<String> list3 = <String>[];
@@ skipping 236 matching lines @@
     testSkipTake(collection, 0, 15);
     testSkipTake(collection, 0, 25);
     testSkipTake(collection, 5, 0);
     testSkipTake(collection, 5, 5);
     testSkipTake(collection, 5, 10);
     testSkipTake(collection, 5, 20);
     testSkipTake(collection, 15, 0);
     testSkipTake(collection, 15, 5);
     testSkipTake(collection, 20, 0);
     testSkipTake(collection, 20, 5);
+    Expect.throws(() => longList.skip(-1), (e) => e is RangeError);
   }
 }