Adds a new Luci test shard that runs flutter/plugins tests.

For now only the analysis phase of the flutter/plugins test suite is executed.

Manual invocation on Luci completed successfully: https://luci-milo.appspot.com/raw/build/logs.chromium.org/flutter/led/amirha_google.com/3210bb2dac01fca71b7db04dda02fa6ccd4df025afd55abea1970d26087d4962/+/annotations