State that skip/take does not accept negative arguments.

sdk/lib/core/iterable.dart

Index: sdk/lib/core/iterable.dart
diff --git a/sdk/lib/core/iterable.dart b/sdk/lib/core/iterable.dart
index 41972b9a255f1339d1f3e510649d943d527c7706..aec73a5d8423bce59ee1093bb9b80890883fca45 100644
--- a/sdk/lib/core/iterable.dart
+++ b/sdk/lib/core/iterable.dart
@@ -171,6 +171,8 @@ abstract class Iterable<E> {
    *
    * The returned [Iterable] may contain fewer than [n] elements, if `this`
    * contains fewer than [n] elements.
+   *
+   * It is an error if [n] is negative.
    */
   Iterable<E> take(int n);
 
@@ -191,6 +193,8 @@ abstract class Iterable<E> {
    *
    * If `this` has fewer than [n] elements, then the resulting [Iterable] will
    * be empty.
+   *
+   * It is an error if [n] is negative.
    */
   Iterable<E> skip(int n);