While trying to add all relevant information to the question I found the cause and a solution:
I changed
<!-- BUG: https://code.google.com/p/dart/issues/detail?id=14382 -->
<!-- link rel="stylesheet" href="xyz.css" -->
<style>
@import url("../../../packages/spark_widgets/spark_modal/spark_modal.css");
</style>
to
<link rel="stylesheet" href="spark_modal.css">
and the error message went away and the examples are working fine now.