Port sky-scrollable to Dart

sky/examples/example-scrollable.sky

-#!mojo mojo:sky_viewer
-<!--
-// 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.
--->
-<sky>
-<import src="/sky/framework/sky-element/sky-element.sky" as="SkyElement" />
-<import src="/sky/framework/sky-scrollable.sky" />
-<import src="data/cities.sky" as="cities" />
-
-<sky-element name="example-scrollable">
-<template>
-  <style>
-  sky-scrollable {
-    margin: 20px;
-    height: 400px;
-    border: 2px solid blue;
-  }
-  b {
-    display: inline;
-    font-weight: bold;
-  }
-  </style>
-  <sky-scrollable>
-    <template repeat="{{ cities }}">
-      <template>
-        <div>{{ name }}</div>
-      </template>
-    </template>
-  </sky-scrollable>
-</template>
-<script>
-module.exports = class extends SkyElement {
-  created() {
-    this.cities = cities.slice(0, 300);
-  }
-}.register();
-</script>
-</sky-element>
-
-<example-scrollable />
-</sky>