introduce web_components pkg for consolidated polyfills

pkg/custom_element/lib/polyfill.dart

-// Copyright (c) 2013, 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.
-
-/** Dart APIs for interacting with the JavaScript Custom Elements polyfill. */
-library custom_element.polyfill;
-
-import 'dart:async';
-import 'dart:html';
-import 'dart:js' as js;
-
-/**
- * A future that completes once all custom elements in the initial HTML page
- * have been upgraded.
- *
- * This is needed because the native implementation can update the elements
- * while parsing the HTML document, but the custom element polyfill cannot,
- * so it completes this future once all elements are upgraded.
- */
-Future customElementsReady = () {
-  if (_isReady) return new Future.value();
-
-  // Not upgraded. Wait for the polyfill to fire the WebComponentsReady event.
-  // Note: we listen on document (not on document.body) to allow this polyfill
-  // to be loaded in the HEAD element.
-  return document.on['WebComponentsReady'].first;
-}();
-
-// Return true if we are using the polyfill and upgrade is complete, or if we
-// have native document.register and therefore the browser took care of it.
-// Otherwise return false, including the case where we can't find the polyfill.
-bool get _isReady {
-  // If we don't have dart:js, assume things are ready
-  if (js.context == null) return true;
-
-  var customElements = js.context['CustomElements'];
-  if (customElements == null) {
-    // Return true if native document.register, otherwise false.
-    // (Maybe the polyfill isn't loaded yet. Wait for it.)
-    return document.supportsRegister;
-  }
-
-  return customElements['ready'] == true;
-}
-
-/**
- * Loads `custom-elements.debug.js` or `custom-elements.min.js` by adding the
- * script tag to the page. Returns a future that completes when custom elements
- * are ready (equivalent to [customElementsReady]).
- *
- * Normally you should add this to your HTML file
- * (the Polymer package will do this automatically), but loading dynamically
- * can be useful for scenarios such as tests.
- */
-Future loadCustomElementPolyfill() {
-  if (!document.supportsRegister && !js.context.hasProperty('CustomElements')) {
-    var script = new ScriptElement()
-        ..src = '/packages/custom_element/custom-elements.debug.js';
-    document.head.append(script);
-    return document.on['WebComponentsReady'].first;
-  }
-  return new Future.value();
-}