Even when design tools can generate code, they don't generate the code you probably want. On the desktop side, there have been more aggressive attempts to allow for code ahead to co exist with the code behind (e.g. XAML), but even then if any kind of abstraction is involved, hand written code will win in the end. There has been more work in academia about round tripping between code, and direct manipulable output (e.g. see Ravi Chugh's work).

Anyways, I'm a strong believer that design and prototyping should be firewalled off from production. Even when the assets are transferrable, the iteration speed demanded by the design process means they'll necessarily lack the polish of production assets.