introduce web_components pkg for consolidated polyfills

pkg/html_import/lib/src/boot.js

-/*
- * Copyright 2013 The Polymer Authors. All rights reserved.
- * Use of this source code is governed by a BSD-style
- * license that can be found in the LICENSE file.
- */
-(function(){
-
-// bootstrap
-
-// IE shim for CustomEvent
-if (typeof window.CustomEvent !== 'function') {
-  window.CustomEvent = function(inType) {
-     var e = document.createEvent('HTMLEvents');
-     e.initEvent(inType, true, true);
-     return e;
-  };
-}
-
-function bootstrap() {
-  // preload document resource trees
-  HTMLImports.importer.load(document, function() {
-    HTMLImports.parser.parse(document);
-    HTMLImports.readyTime = new Date().getTime();
-    // send HTMLImportsLoaded when finished
-    document.dispatchEvent(
-      new CustomEvent('HTMLImportsLoaded', {bubbles: true})
-    );
-  });
-};
-
-if (document.readyState === 'complete') {
-  bootstrap();
-} else {
-  window.addEventListener('DOMContentLoaded', bootstrap);
-}
-
-})();