code: @Override
    public void setStartingPosition(int offsetLine, int offsetIndex) {
      checkState(offsetLine >= 0);
      checkState(offsetIndex >= 0);
      offsetPosition = new FilePosition(offsetLine, offsetIndex);
    }
docs: Sets the source code that exists in the buffer for which the
  generated code is being generated. This ensures that the source map
  accurately reflects the fact that the source is being appended to
  an existing buffer and as such, does not start at line 0, position 0
  but rather some other line and position.
  
  @param offsetLine The index of the current line being printed.
  @param offsetIndex The column index of the current character being printed.