DartDoc incorrectly reports number of libraries/types/members as 0

[DartBot]

This issue was originally filed by sammcca...@google.com

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This is always the first line of output:
  Documented 0 libraries, 0 types, and 0 members.

[dgrove]

Set owner to @munificent.
Added Area-DartDoc, Triaged labels.

[johnniwinther]

Fixed by http://codereview.chromium.org/10658031/

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Added Fixed label.