fix an issue with polymer and dart2js treeshaking

When running the app compiled to JS, I get this exception:

    Uncaught Unsupported operation: Can't use 'isNotEmpty' in reflection because it is not included in a @MirrorsUsed annotation.

which was actually very useful in tracking down the issue. We were using a string method via reflection in a polymer expression, and that method was being tree-shaken (or minified?) out of existence. This changes to calling a `@reflectable` getter in from the html file to make sure everything makes it safely past the dart2js compilation stage.

@ussuri