Rationalize Dart mojo and sky package structure

sky/framework/elements/sky-scrollable.sky

-<!--
-// 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 src="sky-element.sky" />
-
-<sky-element>
-<template>
-  <style>
-  :host {
-    overflow: hidden;
-    position: relative;
-    will-change: transform;
-  }
-  #scrollable {
-    will-change: transform;
-  }
-  #vbar {
-    position: absolute;
-    right: 0;
-    background-color: lightgray;
-    pointer-events: none;
-    top: 0;
-    height: 0;
-    will-change: opacity;
-    opacity: 0;
-    transition-property: opacity;
-    transition-function: ease-in-out;
-  }
-  </style>
-  <div id="scrollable">
-    <content />
-  </div>
-  <div id="vbar" />
-</template>
-<script>
-import "dart:math" as math;
-import "dart:sky";
-import "../animation/fling_curve.dart";
-import "../theme/view-configuration.dart" as config;
-
-@Tagname('sky-scrollable')
-class SkyScrollable extends SkyElement {
-  Element _scrollable;
-  Element _vbar;
-  double _scrollOffset = 0.0;
-  FlingCurve _flingCurve;
-  int _flingAnimationId;
-
-  SkyScrollable() {
-    addEventListener('gesturescrollstart', _handleScrollStart);
-    addEventListener('gesturescrollend', _handleScrollEnd);
-    addEventListener('gesturescrollupdate', _handleScrollUpdate);
-    addEventListener('gestureflingstart', _handleFlingStart);
-    addEventListener('gestureflingcancel', _handleFlingCancel);
-    addEventListener('wheel', _handleWheel);
-  }
-
-  void shadowRootReady() {
-    _scrollable = shadowRoot.getElementById('scrollable');
-    _vbar = shadowRoot.getElementById('vbar');
-    // This is not documented anywhere, but the scrollbar appears to only paint
-    // 3px even though it's official width is 10px?
-    // Chrome appears 3px wide with a 3px outer spacing.
-    // Contacts appears 3px wide with a 5px runner and 5px outer spacing.
-    // Settings appears 4px wide with no outer spacing.
-    const double paintPercent = 0.3;
-    const double outerGapPercent = 0.3;
-    const double innerGapPercent = 0.4;
-    const double paintWidth = paintPercent * config.kScrollbarSize;
-    _vbar.style['width'] = "${paintWidth}px";
-    _vbar.style['margin-right'] = "${outerGapPercent * config.kScrollbarSize}px";
-    _vbar.style['margin-left'] ="${innerGapPercent * config.kScrollbarSize}px";
-    // The scroll thumb never quite makes it to the top or bottom in gmail
-    // or chrome (in chrome more from the bottom than the top).
-    _vbar.style['margin-top'] = "${config.kScrollbarSize}px";
-    _vbar.style['margin-bottom'] = "${config.kScrollbarSize}px";
-
-    // Some android apps round their scrollbars, some don't, not rounding for now.
-
-    const double msToSeconds = 1.0 / 1000.0;
-    _vbar.style['transition-duration'] = "${msToSeconds * config.kScrollbarFadeDuration}s";
-    _vbar.style['transition-delay'] = "${msToSeconds * config.kScrollbarFadeDelay}s";
-  }
-
-  double get scrollOffset => _scrollOffset;
-
-  set scrollOffset(double value) {
-    // TODO(abarth): Can we get these values without forcing a synchronous layout?
-    double outerHeight = clientHeight.toDouble();
-    double innerHeight = _scrollable.clientHeight.toDouble();
-    double scrollRange = innerHeight - outerHeight;
-    double newScrollOffset = math.max(0.0, math.min(scrollRange, value));
-    if (newScrollOffset == _scrollOffset)
-      return;
-    // TODO(eseidel): We should scroll in device pixels instead of logical
-    // pixels, but to do that correctly we need to use a device pixel unit.
-    _scrollOffset = newScrollOffset;
-    String transform = 'translateY(${(-_scrollOffset).toInt()}px)';
-    _scrollable.style['transform'] = transform;
-
-    double topPercent = newScrollOffset / innerHeight * 100.0;
-    double heightPercent = outerHeight / innerHeight * 100.0;
-    _vbar.style['top'] = '${topPercent}%';
-    _vbar.style['height'] = '${heightPercent}%';
-  }
-
-  bool scrollBy(double scrollDelta) {
-    double oldScrollOffset = _scrollOffset;
-    scrollOffset += scrollDelta;
-    return _scrollOffset != oldScrollOffset;
-  }
-
-  void _scheduleFlingUpdate() {
-    _flingAnimationId = window.requestAnimationFrame(_updateFling);
-  }
-
-  void _stopFling() {
-    window.cancelAnimationFrame(_flingAnimationId);
-    _flingCurve = null;
-    _flingAnimationId = null;
-    _vbar.style['opacity'] = '0';
-  }
-
-  void _updateFling(double timeStamp) {
-    double scrollDelta = _flingCurve.update(timeStamp);
-    if (scrollDelta == 0.0 || !scrollBy(scrollDelta))
-      _stopFling();
-    else
-      _scheduleFlingUpdate();
-  }
-
-  void _handleScrollStart(_) {
-    _vbar.style['opacity'] = '1';
-  }
-
-  void _handleScrollEnd(_) {
-    _vbar.style['opacity'] = '0';
-  }
-
-  void _handleScrollUpdate(GestureEvent event) {
-    scrollBy(-event.dy);
-  }
-
-  void _handleFlingStart(GestureEvent event) {
-    _flingCurve = new FlingCurve(-event.velocityY, event.timeStamp);
-    _scheduleFlingUpdate();
-  }
-
-  void _handleFlingCancel(_) {
-    _stopFling();
-  }
-
-  void _handleWheel(WheelEvent event) {
-    scrollBy(-event.offsetY);
-  }
-}
-
-_init(script) => register(script, SkyScrollable);
-</script>
-</sky-element>