Stop using deprecated ContextLocator and ContextBuilder.

[scheglov]

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[devoncarew]

Hmm, the version resolution here seems complicated:

https://github.com/dart-lang/tools/actions/runs/9963959255/job/27531185159?pr=285

[devoncarew]

(we should see if we can get the CI here green before landing)

[scheglov]

I think currently the analyzer requires the latest Dart SDK, because of macros.
And dispose() was potentially necessary because of macros (stop separate isolates running macros).
But practically we don't resolve anything here, just parse, so get get away with not disposing.