code: public static <T> void propagateResult(final Promise<T> source, final Settable<T> dest) {
      source.addListener(new TransformingPromiseListener<T, T>(dest, PromiseTransformer.identity()));
    }
docs: Copies the value or error from the source promise to the destination
  promise.
  
  @param source the source promise
  @param dest the destination promise
  @param <T> the value type for both promises