v 2.1.0 does not preserve whitespace in <pre> blocks

[keertip]

Upgraded to 2.1.0 from 2.0.3 and see a test failure in dartdoc with text like

Sample class [String]\n<pre> A\n B\n</pre>

markdown removes the whitespace before A and B, using HtmlRenderer().render.

[srawlins]

What did that source use to render? What does it render now? Using the command-line tool, it looks like it just puts the source between <p> and </p>, w/o parsing any markdown between:

$ dart bin/markdown.dart
Sample class [String]\n<pre> A\n B\n</pre>^D
<p>Sample class [String]\n<pre> A\n B\n</pre></p>

[keertip]

Passing in a node that's text <pre>\n A\n B\n</pre>, get back string <pre>A\nB\n</pre>

From the test

 Expected: '<p>Sample class <code>String</code></p><pre class="language-dart">  A\n'
              '   B\n'
              '</pre>'
    Actual: '<p>Sample class <code>String</code></p><pre class="language-dart">A\n'
              'B\n'
              '</pre>'
     Which: is different.
            Expected: ... age-dart">  A\n   B\ ...
              Actual: ... age-dart">A\nB\n</pr ...
                                    ^
             Differ at offset 66