Add List.shuffle().

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index e16cbef904c657979f7124d82b9b251a687e3adb..6582248e3dc6150b9ef33254d81a469d41d0ce92 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -202,6 +202,10 @@ class _ObjectArray<E> implements List<E> {
     IterableMixinWorkaround.sortList(this, compare);
   }
 
+  void shuffle() {
+    IterableMixinWorkaround.shuffleList(this);
+  }
+
   int indexOf(Object element, [int start = 0]) {
     return Arrays.indexOf(this, element, start, this.length);
   }
@@ -456,6 +460,11 @@ class _ImmutableArray<E> implements List<E> {
         "Cannot modify an immutable array");
   }
 
+  void shuffle() {
+    throw new UnsupportedError(
+        "Cannot modify an immutable array");
+  }
+
   String toString() {
     return IterableMixinWorkaround.toStringIterable(this, '[', ']');
   }