code: @Override
    public void setWrapperPrefix(String prefix) {
      // Determine the current line and character position.
      int prefixLine = 0;
      int prefixIndex = 0;
  
      for (int i = 0; i < prefix.length(); ++i) {
        if (prefix.charAt(i) == '\n') {
          prefixLine++;
          prefixIndex = 0;
        } else {
          prefixIndex++;
        }
      }
  
      prefixPosition = new FilePosition(prefixLine, prefixIndex);
    }
docs: Sets the prefix used for wrapping the generated source file before
  it is written. This ensures that the source map is adjusted for the
  change in character offsets.
  
  @param prefix The prefix that is added before the generated source code.