Make List.shuffle take an optional Random object to use.

sdk/lib/_collection_dev/list.dart

Index: sdk/lib/_collection_dev/list.dart
diff --git a/sdk/lib/_collection_dev/list.dart b/sdk/lib/_collection_dev/list.dart
index 6acc5d27af906f349f994c73cc78b03edb278360..75486bffbc5349d425a31f37cdcac374197be864 100644
--- a/sdk/lib/_collection_dev/list.dart
+++ b/sdk/lib/_collection_dev/list.dart
@@ -140,7 +140,7 @@ abstract class UnmodifiableListMixin<E> implements List<E> {
         "Cannot modify an unmodifiable list");
   }
 
-  void shuffle() {
+  void shuffle([Random random]) {
     throw new UnsupportedError(
         "Cannot modify an unmodifiable list");
   }