I can't reproduce your problem.
This works for me:
lib/foo.dart
library foo;
import 'package:web_ui/web_ui.dart';
class FooComponent extends WebComponent {
var foo = 'asddfg';
}
lib/foo.html
<element name="x-foo" constructor="FooComponent" extends="div">
<template>
<input value="{{ foo }}" />
</template>
<script type="application/dart" src="foo.dart"></script>
</element>
example/test.html
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<link rel="components" href="packages/foo/foo.html" />
</head>
<body>
<x-foo></x-foo>
<script type="application/dart">main() {}</script>
<script type="text/javascript" src="https://dart.googlecode.com/svn/branches/bleeding_edge/dart/client/dart.js"></script>
</body>
</html>
Then I open up Dartium and go to .../example/out/test.html
and I see an input with the data bound to it properly.