Do not crash in the presence of macro applications.

[srawlins]

Fixes #3653

• Dispose of an AnalysisContextCollectionImpl.
• Before getting class hierarchy, navigate to possible augmentation declaration.
• Do not attempt to access source code of augmentation elements.

I cannot add tests yet because dartdoc needs to depend on a pinned version of the analyzer which matches the SDK in which the tests are run. We could do this if dartdoc was in the SDK repo, and we will more likely start when the macro API stuff is moved to a dart: library.

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