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

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index 5bef0d0db28ec84e79e2c396c37e0822799c5a66..e63af507ca8090efd78afc038cc60d47ca3de902 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -92,7 +92,8 @@ class _ObjectArray<E> implements List<E> {
     return this.length == 0;
   }
 
-  void sort([Comparator<E> compare = Comparable.compare]) {
+  void sort([int compare(E a, E b)]) {
+    if (compare == null) compare = Comparable.compare;
     _Sort.sort(this, compare);
   }
 
@@ -238,7 +239,7 @@ class _ImmutableArray<E> implements List<E> {
     return this.length == 0;
   }
 
-  void sort([Comparator<E> compare]) {
+  void sort([int compare(E a, E b)]) {
     throw new UnsupportedError(
         "Cannot modify an immutable array");
   }