Fix move() to call markNeedsLayout().

Otherwise, changing the order of your children (e.g. shuffling a list of
children) will have no visible effect.

packages/flutter/lib/src/rendering/object.dart

@@ -1968,6 +1968,7 @@ abstract class ContainerRenderObjectMixin<ChildType extends RenderObject, Parent
       return;
     _removeFromChildList(child);
     _insertIntoChildList(child, after: after);
+    markNeedsLayout();
   }
 
   void attach() {

packages/flutter/test/rendering/mutations_test.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:test/test.dart';
+
+import 'rendering_tester.dart';
+
+class RenderLayoutTestBox extends RenderProxyBox {
+  RenderLayoutTestBox(this.onLayout);
+  final VoidCallback onLayout;
+  void layout(Constraints constraints, { bool parentUsesSize: false }) {
+    // Doing this in tests is ok, but if you're writing your own
+    // render object, you want to override performLayout(), not
+    // layout(). Overriding layout() would remove many critical
+    // performance optimizations of the rendering system, as well as
+    // many bypassing many checked-mode integrity checks.
+    super.layout(constraints, parentUsesSize: parentUsesSize);
+    onLayout();
+  }
+  bool get sizedByParent => true;
+  void performLayout() { }
+}
+
+void main() {
+  test('moving children', () {
+    RenderBox child1, child2;
+    bool movedChild1 = false;
+    bool movedChild2 = false;
+    RenderFlex block = new RenderFlex();
+    block.add(child1 = new RenderLayoutTestBox(() { movedChild1 = true; }));
+    block.add(child2 = new RenderLayoutTestBox(() { movedChild2 = true; }));
+
+    expect(movedChild1, isFalse);
+    expect(movedChild2, isFalse);
+    layout(block);
+    expect(movedChild1, isTrue);
+    expect(movedChild2, isTrue);
+
+    movedChild1 = false;
+    movedChild2 = false;
+
+    expect(movedChild1, isFalse);
+    expect(movedChild2, isFalse);
+    pumpFrame();
+    expect(movedChild1, isFalse);
+    expect(movedChild2, isFalse);
+
+    block.move(child1, after: child2);
+    expect(movedChild1, isFalse);
+    expect(movedChild2, isFalse);
+    pumpFrame();
+    expect(movedChild1, isTrue);
+    expect(movedChild2, isTrue);
+
+    movedChild1 = false;
+    movedChild2 = false;
+
+    expect(movedChild1, isFalse);
+    expect(movedChild2, isFalse);
+    pumpFrame();
+    expect(movedChild1, isFalse);
+    expect(movedChild2, isFalse);
+  });
+}