Leading whitespaces are not removed for not the first line of a doc comment

[sgrekhov]

According to the Dart doc comment specification:

For each line that begins with ///, the parser removes the three slashes and all leading whitespace to produce the documentation text.

But in the case when there is at least one doc comment above the leading whitespaces are not removed. For example:

///  Text.
///    More text.

The output produced by the dartdoc is:

Text
  More text.

Link to the failing test will be added later after landing of the PR containig the test.