update hello, world sample and remove slider

samples/slider/slider_sample.dart

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-library slider_sample;
-
-import 'dart:html';
-import '../ui_lib/view/view.dart';
-
-const menuItems = const["Apple", "Banana", "Cherry", "Durian"];
-
-main() {
-  var sliderMenu = new SliderMenu(menuItems, (selectedText) {
-    displaySelection(selectedText);
-  });
-  query("#menu").nodes.add(sliderMenu.node);
-
-  query('#next').on.click.add((e) {
-    sliderMenu.selectNext(true);
-  });
-
-  query('#prev').on.click.add((e) {
-    sliderMenu.selectPrevious(true);
-  });
-
-  document.on.keyDown.add((KeyboardEvent event) {
-    switch (event.keyIdentifier) {
-      case KeyName.LEFT:
-        sliderMenu.selectPrevious(true);
-        break;
-      case KeyName.RIGHT:
-        sliderMenu.selectNext(true);
-        break;
-    }
-  });
-
-  sliderMenu.enterDocument();
-}
-
-void displaySelection(String selectedText) {
-  query("#notes").text = "Selection: ${selectedText}";
-}