Practically remove boot.js, adds the initialization from the Dart side of

pkg/polymer/lib/src/boot.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.
+
+/** Ported from `polymer/src/boot.js`. **/
+part of polymer;
+
+/** Prevent a flash of unstyled content. */
+preventFlashOfUnstyledContent() {
+  var style = new StyleElement();
+  style.text = r'body {opacity: 0;}';
+  // Note: we use `query` and not `document.head` to make sure this code works
+  // with the shadow_dom polyfill (a limitation of the polyfill is that it can't
+  // override the definitions of document, document.head, or document.body).
+  var head = query('head');
+  head.insertBefore(style, head.firstChild);
+
+  Polymer.onReady.then((_) {
+    document.body.style.transition = 'opacity 0.3s';
+    document.body.style.opacity = '1';
+  });
+}