migration tool reanalyzes all `part`s in a library every time it analyzes one

[jcollins-g]

In the debugger, I have seen AnalysisDriver._computeAnalysisResult calls with withUnit = true for every file in a multi-part library, which appears to force re-analysis of the full library. Exactly why we are doing this and what we might do instead is TBD. This is a big reason migration of web_ui is so slow.

[bwilkerson]

If you're using an AnalysisSession to perform analysis, then you can use getSourceKind to determine whether a file is a part. If you then use getResolvedLibrary for libraries and iterate over the already analyzed parts, you can keep track of which part's you didn't find a library for and analyze only those at the end (or signal an error if that's more appropriate).

[jcollins-g]

The pipeline migration uses is somewhat different. The dartfix (shared with dartdev) pipeline uses AbstractAnalysisServer.getResolvedUnit recursively over a file tree built with Resource.getChildren(). The way the resolved units are processed is going to be challenging to build a similar fix for as a lot of context information isn't in the right place for us to dedupe the getResolvedUnit calls.... at least, so far. Still looking.

[jcollins-g]

https://dart-review.googlesource.com/c/sdk/+/138940 now has a proposed fix.