Make List an abstract class.

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.
 
 /**
  * A [List] is an indexable collection with a length. It can be of
  * fixed size or extendable.
  */
-interface List<E> extends Collection<E>, Sequence<E>
-                  default _ListImpl<E> {
+abstract class List<E> implements Collection<E>, Sequence<E> {
   /**
    * Creates a list of the given [length].
    *
    * If no [length] argument is supplied an extendable list of
    * length 0 is created.
    *
    * If a [length] argument is supplied, a fixed size list of that
    * length is created.
    */
-  List([int length]);
+  external factory List([int length]);
 
   /**
    * Creates a list with the elements of [other]. The order in
    * the list will be the order provided by the iterator of [other].
    */
-  List.from(Iterable<E> other);
+  factory List.from(Iterable<E> other) {
+    var list = new List<E>();
+    for (var e in other) {
+      list.add(e);
+    }
+    return list;
+  }
 
   /**
    * Returns the element at the given [index] in the list or throws
    * an [RangeError] if [index] is out of bounds.
    */
   E operator [](int index);
 
   /**
    * Sets the entry at the given [index] in the list to [value].
    * Throws an [RangeError] if [index] is out of bounds.
@@ skipping 136 matching lines @@
    * not extendable.
    * If [length] is 0, this method does not do anything.
    * If [start] is the length of the list, this method inserts the
    * range at the end of the list.
    * Throws an [ArgumentError] if [length] is negative.
    * Throws an [RangeError] if [start] is negative or if
    * [start] is greater than the length of the list.
    */
   void insertRange(int start, int length, [E initialValue]);
 }
-
-class _ListImpl<E> {
-  /**
-   * Factory implementation of List().
-   *
-   * Creates a list of the given [length].
-   */
-  external factory List([int length]);
-
-  /**
-   * Factory implementation of List.from().
-   *
-   * Creates a list with the elements of [other]. The order in
-   * the list will be the order provided by the iterator of [other].
-   */
-  external factory List.from(Iterable<E> other);
-}