Add list.removeAt method.

lib/core/list.dart

Index: lib/core/list.dart
diff --git a/lib/core/list.dart b/lib/core/list.dart
index f040c95546b968afbc2e4c8da9b4cf8419612bdf..83a1c0238de3c40758d22ea148f8e937f58fcfb0 100644
--- a/lib/core/list.dart
+++ b/lib/core/list.dart
@@ -95,10 +95,22 @@ interface List<E> extends Collection<E> default ListImplementation<E> {
    */
   void clear();
 
+  /**  
+   * Removes the element at position[index] from the list.

[ngeoffray, 2012/09/20 09:16:24]

FYI: we did not have removeAt initially because it makes it too simple to use,
without showing that this is an expensive operation. So we have removeRange
already that does removeAt, but with the 'Range' in the name, it *feels*
expensive.

[Lasse Reichstein Nielsen, 2012/09/20 10:50:29]

Performance through inconvenience :)
I don't think it's going to work in the long run. I'd rather provide the methods
that people want to call than make it hard to get the job done (even if it's
done slowly).

+   *
+   * This reduces the length of the list by one and moves
+   * all later elements down by one position.
+   * Returns the removed element.
+   * Throws a [UnsupportedOperationException] if the list, or the 
+   * length of the list, cannot be changed.
+   */
+  E removeAt(int index);
+
   /**
    * Pops and returns the last element of the list.
    * Throws a [UnsupportedOperationException] if the length of the
    * list cannot be changed.
+   * 
    */
   E removeLast();