Add "contains" method to Collection.

lib/core/list.dart

Index: lib/core/list.dart
diff --git a/lib/core/list.dart b/lib/core/list.dart
index e44e743974677c650ce08f4dc1621155ec264c06..77f6514c12095d98316ef0c5926571d398a3c777 100644
--- a/lib/core/list.dart
+++ b/lib/core/list.dart
@@ -6,8 +6,8 @@
  * 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> {
-
+interface List<E> extends Collection<E>, Sequence<E>
+                  default ListImplementation<E> {
   /**
    * Creates a list of the given [length].
    *