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

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index 75a674f745f8a6eef678df80a9694b6a8a8fa590..9df153d9f11eff082f636123239ae2ad48bac89a 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -92,7 +92,7 @@ class _ObjectArray<E> implements List<E> {
     return this.length === 0;
   }
 
-  void sort(int compare(E a, E b)) {
+  void sort([Comparator<E> compare = Comparable.compare]) {
     DualPivotQuicksort.sort(this, compare);
   }
 
@@ -234,7 +234,7 @@ class _ImmutableArray<E> implements List<E> {
     return this.length === 0;
   }
 
-  void sort(int compare(E a, E b)) {
+  void sort([Comparator<E> compare]) {
     throw const UnsupportedOperationException(
         "Cannot modify an immutable array");
   }