Fix summary generation in "package-summary-only" mode.
Fix summary generation in "package-summary-only" mode.

If we aren't doing full analysis, then we can't use
`context.librarySources` to find out the set of library sources (because
it doesn't include sources that haven't been examined yet).  Instead, we
have to query each source to find out its type.

R=scheglov@google.com

Review URL: https://codereview.chromium.org/1770323003 .

dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk