Apply a change to the document.

def apply_change(self, change):
        """Apply a change to the document."""
        text = change['text']
        change_range = change.get('range')

        if not change_range:
            # The whole file has changed
            self._source = text
            return

        start_line = change_range['start']['line']
        start_col = change_range['start']['character']
        end_line = change_range['end']['line']
        end_col = change_range['end']['character']

        # Check for an edit occuring at the very end of the file
        if start_line == len(self.lines):
            self._source = self.source + text
            return

        new = io.StringIO()

        # Iterate over the existing document until we hit the edit range,
        # at which point we write the new text, then loop until we hit
        # the end of the range and continue writing.
        for i, line in enumerate(self.lines):
            if i < start_line:
                new.write(line)
                continue

            if i > end_line:
                new.write(line)
                continue

            if i == start_line:
                new.write(line[:start_col])
                new.write(text)

            if i == end_line:
                new.write(line[end_col:])

        self._source = new.getvalue()