function makeBlock(isShadow) {
    return {
      isShadow: function () {
        return isShadow;
      },
    };
  }
Make a block stub that implements only `isShadow` and returns the given
value.
@param {boolean} isShadow Whether `isShadow` should return true or false.
@returns {!Object} A block stub.
@package