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

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index fe9a15b9f61a2a2f1146bc82531ce2c8bf5b8c8a..a953f71ea3f1fd6c675a2c3ca7c7a4f16ba618e4 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -202,8 +202,8 @@ class _List<E> implements List<E> {
     IterableMixinWorkaround.sortList(this, compare);
   }
 
-  void shuffle() {
-    IterableMixinWorkaround.shuffleList(this);
+  void shuffle([Random random]) {
+    IterableMixinWorkaround.shuffleList(this, random);
   }
 
   int indexOf(Object element, [int start = 0]) {
@@ -460,7 +460,7 @@ class _ImmutableList<E> implements List<E> {
         "Cannot modify an immutable array");
   }
 
-  void shuffle() {
+  void shuffle([Random random]) {
     throw new UnsupportedError(
         "Cannot modify an immutable array");
   }