State that skip/take does not accept negative arguments.

tests/corelib/iterable_take_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 197 matching lines @@
   it = take0.iterator;
   Expect.isNull(it.current);
   Expect.isFalse(it.moveNext());
   Expect.isNull(it.current);
 
   take1 = set2.take(1);
   it = take1.iterator;
   Expect.isNull(it.current);
   Expect.isFalse(it.moveNext());
   Expect.isNull(it.current);
+
+  Expect.throws(() => list1.skip(-1), (e) => e is RangeError);
+  Expect.throws(() => list2.skip(-1), (e) => e is RangeError);
+  Expect.throws(() => list3.skip(-1), (e) => e is RangeError);
+  Expect.throws(() => set1.skip(-1), (e) => e is RangeError);
+  Expect.throws(() => set2.skip(-1), (e) => e is RangeError);
+  Expect.throws(() => list1.map((x) => x).skip(-1), (e) => e is RangeError);
+  Expect.throws(() => list2.map((x) => x).skip(-1), (e) => e is RangeError);
+  Expect.throws(() => list3.map((x) => x).skip(-1), (e) => e is RangeError);
+  Expect.throws(() => set1.map((x) => x).skip(-1), (e) => e is RangeError);
+  Expect.throws(() => set2.map((x) => x).skip(-1), (e) => e is RangeError);
 }