This is code that was valid in Polymer about ">= 0.10.5 <0.11.0"
but there were several changes.
;component=1
Should be remove from all script tags.
The imports and script tags in the entry page should look like
<html>
<head>
<title>core-ajax-dart</title>
<!-- when the project uses polymer -->
<!-- <script src="packages/web_components/platform.js"></script>
not necessary anymore with Polymer >= 0.14.0 -->
<script src="packages/web_components/dart_support.js"></script>
<!-- import individual polymer elements -->
<link rel='import' href='packages/core_elements/core_ajax_dart.html'>
</head>
<body>
<core-ajax-dart
url="http://gdata.youtube.com/feeds/api/videos/"
params='{"alt":"json", "q":"chrome"}'
handleAs="json"
auto></core-ajax-dart>
<!-- if you have a custom script file that contains a main() method -->
<script type="application/dart" src="core_ajax_dart.dart"></script>
<!-- else if you don't have a custom script file with a main() method
<script type="application/dart">export 'package:polymer/init.dart';</script>
</body>
</html>
see here how to implement a custom main method.
how to implement a main function in polymer apps