Make argument of List.sort optional with default Comparable.compare.

lib/core/list.dart

Index: lib/core/list.dart
diff --git a/lib/core/list.dart b/lib/core/list.dart
index e44e743974677c650ce08f4dc1621155ec264c06..d362d0fc66cdfa2a6dac3b07279aa4e7adffe633 100644
--- a/lib/core/list.dart
+++ b/lib/core/list.dart
@@ -69,7 +69,7 @@ interface List<E> extends Collection<E> default ListImplementation<E> {
   /**
    * Sorts the list according to the order specified by the [Comparator].
    */
-  void sort(Comparator<E> compare);
+  void sort([Comparator<E> compare = Comparable.compare]);
 
   /**
    * Returns the first index of [element] in the list.