code: function(key, value) {
          requires(key, 'undefined is not a valid key');
          requires(value, 'undefined is not a valid value');
          this._where[key] = AV._encode(value);
          return this;
        }
docs: Add a constraint to the query that requires a particular key's value to
  be equal to the provided value.
  @param {String} key The key to check.
  @param value The value that the AV.Object must contain.
  @return {AV.Query} Returns the query, so you can chain this call.