I would suggest thinking about this in a slightly different manner: try applying a CSS class using the Dart syntax
<html>
<body>
<element name='foo' constructor='FooElement' extends='div'>
<style>
div {
display: block;
}
div.hide {
display: none;
}
</style>
<template>
<div class="{{isVisible ? '' : 'hide'}}">Foobar</div>
</template>
<script type="application/dart" src="xfooelement.dart"></script>
</element>
</body>
</html>