function _specToFunction(spec) {
  throw new Error('Declarative specs are not implemented yet.')
}
Coerces the given spec into a function which {@link Shim#wrap} can use.
returns WrapFunction The spec itself if spec is a function, otherwise a
   function which will execute the spec when called.

@private
@param {Spec|WrapFunction} spec - The spec to coerce into a function.