In gradle-based builds, we have a FlutterPlugin.groovy in the user's source tree. As we iterate in the build system, we're likely to want to change this plugin. We need a mechanism for updating the copy of this file in the user's source tree when we change the version in our source tree.

The simplest thing is to check whether the version in the user's source tree is byte-identical (or has a matching hash) with the version flutter_tools expects.