Rationalize Dart mojo and sky package structure

sky/framework/components/icon.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 '../fn.dart';
-
-// TODO(eseidel): This should use package:.
-const String kAssetBase = '/packages/sky/assets/material-design-icons';
-
-class Icon extends Component {
-  int size;
-  String type;
-
-  Icon({
-    String key,
-    this.size,
-    this.type: ''
-  }) : super(key: key);
-
-  UINode build() {
-    String category = '';
-    String subtype = '';
-    List<String> parts = type.split('/');
-    if (parts.length == 2) {
-      category = parts[0];
-      subtype = parts[1];
-    }
-
-    return new Image(width: size, height: size,
-      src: '${kAssetBase}/${category}/2x_web/ic_${subtype}_${size}dp.png');
-  }
-}