Move ListImplementation from coreimpl to core, as a private member.

lib/core/list.dart

Index: lib/core/list.dart
diff --git a/lib/core/list.dart b/lib/core/list.dart
index 6dd841fdc728c734eb4ca6151cd66cfd90b39f85..2cb318571adb6ddfa6b9a2de1530276be79ee9eb 100644
--- a/lib/core/list.dart
+++ b/lib/core/list.dart
@@ -7,7 +7,7 @@
  * fixed size or extendable.
  */
 interface List<E> extends Collection<E>, Sequence<E>
-                  default ListImplementation<E> {
+                  default _ListImpl<E> {
   /**
    * Creates a list of the given [length].
    *
@@ -174,3 +174,20 @@ interface List<E> extends Collection<E>, Sequence<E>
    */
   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);
+}