SkipIterable - ensure field is only initialized to a known int.

tests/corelib/iterable_skip_test.dart

Index: tests/corelib/iterable_skip_test.dart
diff --git a/tests/corelib/iterable_skip_test.dart b/tests/corelib/iterable_skip_test.dart
index a06cb6e05289816bc64ad9937720590375ba40f1..8af142bcdf87a3c109929aa8f438fc1094fdf15d 100644
--- a/tests/corelib/iterable_skip_test.dart
+++ b/tests/corelib/iterable_skip_test.dart
@@ -252,6 +252,7 @@ main() {
     testSkipTake(collection, 15, 5);
     testSkipTake(collection, 20, 0);
     testSkipTake(collection, 20, 5);
-    Expect.throws(() => longList.skip(-1), (e) => e is RangeError);
+    Expect.throws(() => collection.skip(-1), (e) => e is RangeError);
+    Expect.throws(() => collection.skip(1).skip(-1), (e) => e is RangeError);
   }
 }