Port of github.com/polymer/polymer.

pkg/polymer/lib/boot.js

Index: pkg/polymer/lib/boot.js
diff --git a/pkg/polymer/lib/boot.js b/pkg/polymer/lib/boot.js
index 0a3fe62baaf390456fa263a261a799f8873919a8..2bcb5902bd085fb5cd76725f2780d10b71b63439 100644
--- a/pkg/polymer/lib/boot.js
+++ b/pkg/polymer/lib/boot.js
@@ -56,8 +56,30 @@
       ' tool to compile a depolyable JavaScript version')
     return;
   }
-  document.write(
-    '<script src="packages/html_import/html_import.min.js"></script>');
+
+
+  // Load HTML Imports:
+  var htmlImportsSrc = 'src="packages/html_import/html_import.min.js"';
+  document.write('<script ' + htmlImportsSrc + '></script>');
+  var importScript = document.querySelector('script[' + htmlImportsSrc + ']');
+  importScript.addEventListener('load', function() {
+    // NOTE: this is from polymer/src/lib/dom.js
+    window.HTMLImports.importer.preloadSelectors +=
+        ', polymer-element link[rel=stylesheet]';
+  });
+
+  // TODO(jmesserly): we need this in deploy tool too.
+  // NOTE: this is from polymer/src/boot.js:
+  // FOUC prevention tactic
+  var style = document.createElement('style');
+  style.textContent = 'body {opacity: 0;}';
+  var head = document.querySelector('head');
+  head.insertBefore(style, head.firstChild);
+
+  window.addEventListener('WebComponentsReady', function() {

[blois, 2013/09/27 21:40:52]

WebComponentsReady comes from the CustomElements polyfill, html imports fire
HTMLImportsLoaded.

Is this the right event? How is the custom elements polyfill being loaded?

[Jennifer Messerly, 2013/09/30 17:44:37]

Yes it is right. As TODO notes this code is copy+paste from polymer/src/boot.js

If you look in the custom element polyfill, the order is HTMLImportsLoaded (if
polyfill, otherwise DOMContentLoaded) followed by WebComponentsReady.

This is kinda wrong right now because custom_element.dart is a very limited
xtags polyfill. It will be right after your change lands to switch Polymer to
real custom elements

+    document.body.style.webkitTransition = 'opacity 0.3s';
+    document.body.style.opacity = 1;
+  });
 
   // Extract a Dart import URL from a script tag, which is the 'src' attribute
   // of the script tag, or a data-url with the script contents for inlined code.