Change List.sort to not have a default parameter value.

runtime/lib/growable_array.dart

Index: runtime/lib/growable_array.dart
diff --git a/runtime/lib/growable_array.dart b/runtime/lib/growable_array.dart
index 2e012c20b720a458611be2dde5a8e0ba51b895eb..066f4b697050fe1f8847680328247a44f0e1cf83 100644
--- a/runtime/lib/growable_array.dart
+++ b/runtime/lib/growable_array.dart
@@ -211,7 +211,8 @@ class _GrowableObjectArray<T> implements List<T> {
     this.length = 0;
   }
 
-  void sort([Comparator<T> compare = Comparable.compare]) {
+  void sort([int compare(T a, T b)]) {
+    if (compare == null) compare = Comparable.compare;
     _Sort.sort(this, compare);
   }