code: function unbind (vm, key) {
    var source = vm._wilddogSources && vm._wilddogSources[key]
    if (!source) {
      throw new Error(
        'VueWild: unbind failed: "' + key + '" is not bound to ' +
        'a Wilddog reference.'
      )
    }
    var listeners = vm._wilddogListeners[key]
    for (var event in listeners) {
      source.off(event, listeners[event])
    }
    vm[key] = null
    vm.$wilddogRefs[key] = null
    vm._wilddogSources[key] = null
    vm._wilddogListeners[key] = null
  }
docs: Unbind a wilddog-bound key from a vm.
  
  @param {Vue} vm
  @param {string} key