[ShadyCSS] Add support for calling prepareTemplate for a given name more than once.

[sorvell]

This should provide better integration with [lit-html'(https://github.com/Polymer/lit-html)s shady-render functionality. Currently, @apply will not correctly track properties when prepareTemplate is called multiple times, losing the previously tracked properties for a given mixin.

[schalkneethling]

I second this. My usage is not in the same context but, not having to dynamically remove and create a new template element between each call to prepareTemplate would be great.

I know the flag _prepared is set on the template element[1] and if this is true the function bails. Perhaps one can pass { force: true } to indicate to the function that it should ignore the value of _prepared and do its thing again? I can see that being useful. Thanks!

[1] https://github.com/webcomponents/shadycss/blob/master/src/scoping-shim.js#L78

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.