State that skip/take does not accept negative arguments.

sdk/lib/core/iterable.dart

 // Copyright (c) 2011, 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.
 
 part of dart.core;
 
 /**
  * The [Iterable] interface allows to get an [Iterator] out of an
  * [Iterable] object.
  *
@@ skipping 153 matching lines @@
   /**
    * Returns true if there is at least one element in this collection.
    */
   bool get isNotEmpty;
 
   /**
    * Returns an [Iterable] with at most [n] elements.
    *
    * 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);
 
   /**
    * Returns an [Iterable] that stops once [test] is not satisfied anymore.
    *
    * The filtering happens lazily. Every new [Iterator] of the returned
    * [Iterable] will start iterating over the elements of `this`.
    *
    * When the iterator encounters an element `e` that does not satisfy [test],
    * it discards `e` and moves into the finished state. That is, it will not
    * ask or provide any more elements.
    */
   Iterable<E> takeWhile(bool test(E value));
 
   /**
    * Returns an [Iterable] that skips the first [n] elements.
    *
    * 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);
 
   /**
    * Returns an [Iterable] that skips elements while [test] is satisfied.
    *
    * The filtering happens lazily. Every new [Iterator] of the returned
    * [Iterable] iterates over all elements of `this`.
    *
    * As long as the iterator's elements satisfy [test] they are
@@ skipping 99 matching lines @@
  */
 abstract class BidirectionalIterator<E> implements Iterator<E> {
   /**
    * Move back to the previous element.
    *
    * Returns true and updates [current] if successful. Returns false
    * and sets [current] to null if there is no previous element.
    */
   bool movePrevious();
 }