Remove the mixed_viewport demo

It's redundant with card_collection.dart, which does similar things but
much better.

examples/widgets/mixed_viewport.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 'dart:math' as math;
-
-import 'package:sky/widgets.dart';
-
-class MixedViewportApp extends App {
-
-  MixedViewportLayoutState layoutState = new MixedViewportLayoutState();
-  List<double> lengths = <double>[];
-  double offset = 0.0;
-
-  static const double kMaxLength = 100.0;
-
-  static math.Random rand = new math.Random();
-
-  void addBox() {
-    lengths.add(rand.nextDouble() * kMaxLength);
-    updateEnabledState();
-  }
-
-  void removeBox(int index) {
-    lengths.removeAt(index);
-    updateEnabledState();
-  }
-
-  void goUp() {
-    offset -= 9.9;
-    updateEnabledState();
-  }
-
-  void goDown() {
-    offset += 20.45;
-    updateEnabledState();
-  }
-
-  bool enabledDown = true;
-  bool enabledUp = true;
-  bool enabledAdd = true;
-  bool enabledRemove = false;
-  void updateEnabledState() {
-    setState(() {
-      enabledUp = offset > -100.0;
-      enabledDown = offset < lengths.fold(0.0, (double result, double len) => result + len) + 100.0;
-      enabledAdd = true;
-      enabledRemove = lengths.length > 0;
-    });
-  }
-
-  Widget build() {
-    return new Theme(
-      data: new ThemeData.light(),
-      child: new Scaffold(
-        toolbar: new ToolBar(
-            center: new Text('Block Viewport Demo')),
-        body: new Material(
-          type: MaterialType.canvas,
-          child: new Column([
-              new Container(
-                padding: new EdgeDims.symmetric(horizontal: 8.0, vertical: 25.0),
-                child: new Row([
-                    new RaisedButton(
-                      enabled: enabledAdd,
-                      child: new Text('ADD'),
-                      onPressed: addBox
-                    ),
-                    new RaisedButton(
-                      enabled: enabledUp,
-                      child: new Text('UP'),
-                      onPressed: goUp
-                    ),
-                    new RaisedButton(
-                      enabled: enabledDown,
-                      child: new Text('DOWN'),
-                      onPressed: goDown
-                    )
-                  ],
-                  justifyContent: FlexJustifyContent.spaceAround
-                )
-              ),
-              new Flexible(
-                child: new Container(
-                  margin: new EdgeDims.all(8.0),
-                  decoration: new BoxDecoration(
-                    border: new Border.all(color: new Color(0xFF000000))
-                  ),
-                  padding: new EdgeDims.all(16.0),
-                  child: new MixedViewport(
-                    builder: builder,
-                    startOffset: offset,
-                    token: lengths.length,
-                    layoutState: layoutState
-                  )
-                )
-              ),
-            ],
-            justifyContent: FlexJustifyContent.spaceBetween
-          )
-        )
-      )
-    );
-  }
-
-  Widget builder(int index) {
-    if (index >= lengths.length)
-      return null;
-    return new GestureDetector(
-      key: new ValueKey<double>(lengths[index]),
-      onTap: () => removeBox(index),
-      child: new Container(
-        decoration: new BoxDecoration(
-          backgroundColor: new Color((0xFF000000 + 0xFFFFFF * lengths[index] / kMaxLength).round())
-        ),
-        height: lengths[index] + 12.0
-      )
-    );
-  }
-
-}
-
-void main() {
-  runApp(new MixedViewportApp());
-}