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

runtime/lib/growable_array.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class _GrowableObjectArray<T> implements List<T> {
   factory _GrowableObjectArray._uninstantiable() {
     throw const UnsupportedOperationException(
         "GrowableObjectArray can only be allocated by the VM");
   }
 
@@ skipping 189 matching lines @@
   }
 
   bool isEmpty() {
     return this.length === 0;
   }
 
   void clear() {
     this.length = 0;
   }
 
-  void sort(int compare(T a, T b)) {
+  void sort([Comparator<T> compare = Comparable.compare]) {
     DualPivotQuicksort.sort(this, compare);
   }
 
   String toString() {
     return Collections.collectionToString(this);
   }
 
   Iterator<T> iterator() {
     return new VariableSizeArrayIterator<T>(this);
   }
@@ skipping 13 matching lines @@
   T next() {
     if (!hasNext()) {
       throw const NoMoreElementsException();
     }
     return _array[_pos++];
   }
 
   final _GrowableObjectArray<T> _array;
   int _pos;
 }