Convert String and List to abstract classes.

lib/core/list.dart

Index: lib/core/list.dart
diff --git a/lib/core/list.dart b/lib/core/list.dart
index e44e743974677c650ce08f4dc1621155ec264c06..1262a6eb0e196d3218a7f06cbaf8353b74435791 100644
--- a/lib/core/list.dart
+++ b/lib/core/list.dart
@@ -6,8 +6,7 @@
  * A [List] is an indexable collection with a length. It can be of
  * fixed size or extendable.
  */
-interface List<E> extends Collection<E> default ListImplementation<E> {
-
+abstract class List<E> extends Collection<E> {
   /**
    * Creates a list of the given [length].
    *
@@ -17,13 +16,19 @@ interface List<E> extends Collection<E> default ListImplementation<E> {
    * 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) {
+    List<E> result = new List<E>();
+    for (E element in other) {
+      result.add(element);
+    }
+    return result;
+  }
 
   /**
    * Returns the element at the given [index] in the list or throws