State that skip/take does not accept negative arguments.

tests/corelib/iterable_take_test.dart

Index: tests/corelib/iterable_take_test.dart
diff --git a/tests/corelib/iterable_take_test.dart b/tests/corelib/iterable_take_test.dart
index eda953a7ca4fc233245bc121e927f5aafbf6d03a..fdedb93e320b0a882826ad8d932ffcfda80fe35c 100644
--- a/tests/corelib/iterable_take_test.dart
+++ b/tests/corelib/iterable_take_test.dart
@@ -215,4 +215,15 @@ main() {
   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);
 }