anchor.diff:    }
   
     int computeUnsignedHeaderChecksum() {
  -    var result = 0;
  -
  -    for (var i = 0; i < length; i++) {
  -      result += (i < checksumOffset || i >= _checksumEnd)
  -          ? this[i] // Not in range of where the checksum is written
  -          : _checksumPlaceholder;
  +    // Accessing the last element first helps the VM eliminate bounds checks in
  +    // the loops below.
  +    this[blockSize - 1];
  +    var result = checksumLength * _checksumPlaceholder;
  +
  +    for (var i = 0; i < checksumOffset; i++) {
  +      result += this[i];
  +    }
  +    for (var i = _checksumEnd; i < blockSize; i++) {
  +      result += this[i];
       }
   
       return result;
     }
   
     int computeSignedHeaderChecksum() {
  -    var result = 0;
  -
  -    for (var i = 0; i < length; i++) {
  -      // Note that _checksumPlaceholder.toSigned(8) == _checksumPlaceholder
  -      result += (i < checksumOffset || i >= _checksumEnd)
  -          ? this[i].toSigned(8)
  -          : _checksumPlaceholder;
  +    this[blockSize - 1];
  +    // Note that _checksumPlaceholder.toSigned(8) == _checksumPlaceholder
  +    var result = checksumLength * _checksumPlaceholder;
  +
  +    for (var i = 0; i < checksumOffset; i++) {
  +      result += this[i].toSigned(8);
  +    }
  +    for (var i = _checksumEnd; i < blockSize; i++) {
  +      result += this[i].toSigned(8);
       }
   
       return result;
  
positive.code:     final $this = this;
      return $this is Uint8List ? $this : Uint8List.fromList(this);
    }
  
    bool get isAllZeroes {
      for (var i = 0; i < length; i++) {
        if (this[i] != 0) return false;
      }
  
      return true;
    }
  }
  
  /// Generates a chunked stream of [length] zeroes.
  
negative.code:   bool matchesHeader(List<int> header, {int offset = magicOffset}) {
      for (var i = 0; i < header.length; i++) {
        if (this[offset + i] != header[i]) return false;
      }
  
      return true;
    }
  
    bool get isAllZeroes {
      for (var i = 0; i < length; i++) {
        if (this[i] != 0) return false;
      }
  
      return true;
  