Fix the check for the expected number of categorized libraries in Dart SDK

[srawlins]

Recently many libraries were deprecated, and the HTML changed sufficiently to break our sanity check that we generate anything when we generate the SDK docs.

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[devoncarew]

I think these changes are expected. Is that right @devoncarew ? I can get a screenshot of the new categorization on the sidebar.

Did something land here? I thought https://dart-review.googlesource.com/c/sdk/+/403387 was still in progress.

[srawlins]

Ah the count difference is just from the libraries being deprecated, and so the HTML doesn't match what that test expects. The newly deprecated libraries are still visible, but srtuck through.

[srawlins]

CC @dart-lang/analyzer-team