code: function _maskContext(context) {
      var Component = this._currentElement.type;
      var contextTypes = Component.contextTypes;
      if (!contextTypes) {
        return emptyObject;
      }
      var maskedContext = {};
      for (var contextName in contextTypes) {
        maskedContext[contextName] = context[contextName];
      }
      return maskedContext;
    }
docs: Filters the context object to only contain keys specified in
  `contextTypes`
  
  @param {object} context
  @return {?object}
  @private