Add List.shuffle().

tools/dom/src/ImmutableListMixin.dart

Index: tools/dom/src/ImmutableListMixin.dart
diff --git a/tools/dom/src/ImmutableListMixin.dart b/tools/dom/src/ImmutableListMixin.dart
index 2c05c11098b7fbb8dee9d051f8754a87d503ba06..36a51865da1d818d09b310bc636b5275a374c6e5 100644
--- a/tools/dom/src/ImmutableListMixin.dart
+++ b/tools/dom/src/ImmutableListMixin.dart
@@ -27,6 +27,10 @@ abstract class ImmutableListMixin<E> implements List<E> {
     throw new UnsupportedError("Cannot sort immutable List.");
   }
 
+  void shuffle() {
+    throw new UnsupportedError("Cannot shuffle immutable List.");
+  }
+
   void insert(int index, E element) {
     throw new UnsupportedError("Cannot add to immutable List.");
   }