Optimize List.toList/.sublist and List.from on lists.

sdk/lib/core/list.dart

 // Copyright (c) 2012, 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;
 
 /**
  * An indexable collection of objects with a length.
  *
  * Subclasses of this class implement different kinds of lists.
@@ skipping 72 matching lines @@
   external factory List.filled(int length, E fill);
 
   /**
    * Creates a list and initializes it using the contents of [other].
    *
    * The [Iterator] of [other] provides the order of the objects.
    *
    * This constructor returns a growable list if [growable] is true;
    * otherwise, it returns a fixed-length list.
    */
-  factory List.from(Iterable other, { bool growable: true }) {
-    List<E> list = new List<E>();
-    for (E e in other) {
-      list.add(e);
-    }
-    if (growable) return list;
-    return makeListFixedLength(list);
-  }
+  external factory List.from(Iterable other, { bool growable: true });
 
   /**
    * Generates a list of values.
    *
    * Creates a list with [length] positions and fills it with values created by
    * calling [generator] for each index in the range `0` .. `length - 1`
    * in increasing order.
    *
    *     new List<int>.generate(3, (int index) => index * index); // [0, 1, 4]
    *
@@ skipping 343 matching lines @@
    * as values. The `Map.keys` [Iterable] iterates the indices of this list
    * in numerical order.
    *
    *     List<String> words = ['fee', 'fi', 'fo', 'fum'];
    *     Map<int, String> map = words.asMap();
    *     map[0] + map[1];   // 'feefi';
    *     map.keys.toList(); // [0, 1, 2, 3]
    */
   Map<int, E> asMap();
 }