Specify the base branch when running "flutter_plugin_tools analyze"

flutter_plugin_tools is now querying FETCH_HEAD by default after the changes in flutter/packages@8c287e9

This causes issues for the analyze_flutter_packages.sh script, which creates a clone of the flutter/packages repository that does not have a FETCH_HEAD.

With the --base-branch flag flutter_plugin_tools will instead query the head of the main branch.

Change-Id: I5aa69b80789fdf85497974cf29d25a66166ddc3e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/418200
Reviewed-by: Slava Egorov <[email protected]>
Commit-Queue: Slava Egorov <[email protected]>

Author: jason-simmons

tools/bots/flutter/analyze_flutter_packages.sh

@@ -41,4 +41,5 @@ cd packages
 dart run script/tool/bin/flutter_plugin_tools.dart analyze \
   --downgrade \
   --analysis-sdk $sdk \
-  --custom-analysis=script/configs/custom_analysis.yaml
+  --custom-analysis=script/configs/custom_analysis.yaml \
+  --base-branch=main