h2 = function(b) {
    return new e2e.hash.Sha256().hash(
        [b].concat(Array.apply([], secbytes.serialize())));
  }
h2 function as defined in OTR spec.
@param {number} b The input byte.
@return {!e2e.ByteArray} The 256-bit output of the SHA256 hash of the
    (5+len) bytes consisting of the byte b followed by secbytes.