function pushByte(b) {
    output += String.fromCharCode(b);
  }
Base64-decode a string into a binary bytestring.  Note that binary strings
are discouraged now that Uint8Array is available on all supported browsers.
Users are encouraged to strongly consider `decodeStringToUint8Array`.  This
method is likely to be deprecated at some point in favor of the Uint8Array
version.

@param {string} input Input to decode. Any whitespace is ignored, and the
    input maybe encoded with either supported alphabet (or a mix thereof).
@param {boolean=} useCustomDecoder True indicates the custom decoder is used,
    which supports alternative alphabets. Note that passing false may still
    use the custom decoder on browsers without native support.
@return {string} string representing the decoded value.