Enable analysis for the tool directory

[stuartmorgan]

Since the tooling doesn't live in packages/, we're not currently analyzing it. This enables analysis so that we won't have analyzer issues creeping in over time.

To minimize complexity, this just adds it directly to the Cirrus configuration rather than building knowledge of the tool directory into the tool itself.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format. See plugin_tool format)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[cyanglaz]

LGTM.
We can possibly move it to "packages" since it is a package now?

[stuartmorgan]

I thought about that, but it's not a plugin, which makes things potentially confusing. (And I don't think we want to move it to flutter/packages for the maintenance reasons we've previously discussed; the heavier use by far is in this repo.)

My thinking was that if it's a one-off, this should be fine, but if we start seeing a pattern of wanting to extend checks to it that are currently package-based we should revisit.

[stuartmorgan]

Landing with red Windows Plugins, as we've temporarily disabled that test to re-open the tree while infrastructure issues with that test bot are worked out.