code: function(component, key) {
      // Memoize the setters.
      var cache = component.__keySetters || (component.__keySetters = {});
      return cache[key] || (cache[key] = createStateKeySetter(component, key));
    }
docs: Returns a single-argument callback that can be used to update a single
  key in the component's state.
  
  Note: this is memoized function, which makes it inexpensive to call.
  
  @param {ReactCompositeComponent} component
  @param {string} key The key in the state that you should update.
  @return {function} callback of 1 argument which calls setState() with
  the provided keyName and callback argument.