auto jsify objects in setters

This is a bit of a speculative cl, but from some basic testing it seems to work well. Are there any cases where doing this by default might cause an issue that I am not thinking of? This is primarily in response to https://github.com/dart-lang/core-elements/issues/57.
I'd move the assignment one level out:
`$body = (value is Map || value is Iterable) ? new JsObject.jsify(value) : value;`