CI broken: `source_gen` needs upgrade (it tries to call a method in `analyzer` that has been removed)

[chrisbobbe]

$ tools/check build_runner
Running build_runner...
  Generating the build script.
  Compiling the build script.
W ../../.pub-cache/hosted/pub.dev/source_gen-4.0.0/lib/src/constants/revive.dart:82:40: Error: The method 'getInvocation' isn't defined for the type 'DartObjectImpl'.
   - 'DartObjectImpl' is from 'package:analyzer/src/dart/constant/value.dart' ('../../.pub-cache/hosted/pub.dev/analyzer-8.4.1/lib/src/dart/constant/value.dart').
  Try correcting the name to the name of an existing method, or defining a method named 'getInvocation'.
    final i = (object as DartObjectImpl).getInvocation();
                                         ^^^^^^^^^^^^^
E Failed to compile build script. Check builder definitions and generated script .dart_tool/build/entrypoint/build.dart.

FAILED: build_runner

To rerun the suites that failed, run:
  $ tools/check build_runner

I'll send a PR for this soon.