Most of our commands are generally of the form:
```
for (each plugin as defined by the tool flags)
check some things for success or failure
print a summary all of the failing things
exit non-zero if anything failed
```
Currently all that logic not consistent, having been at various points copied and pasted around, modified, in some cases rewritten. There's unnecessary boilerplate in each new command, and there's unnecessary variation that makes it harder both to maintain the tool, and to consume the test output:
- There's no standard format for separating each plugin's run to search within a log
- There's no standard format for the summary at the end
- In some cases commands have been written to ToolExit on failure, which means we don't actually get the rest of the runs
This makes a new base class for commands that follow this structure to use, with shared code for all the common bits. This makes it harder to accidentally write new commands incorrectly, easier to maintain the code, and lets us standardize output so that searching within large logs will be easier.
This ports two commands over as a proof of concept to demonstrate that it works; more will be converted in follow-ups.
Related to https://github.com/flutter/flutter/issues/83413
common.dart is a large-and-growing file containing all shared code,
which makes it hard to navigate. To make maintenance easier, this splits
the file (and its test file) into separate files for each major
component or category.
Eliminates the global test filesystem and global test packages directory, in favor of local versions. This guarantees that each test runs with a clean filesystem state, rather than relying on cleanup. It also simplifies understanding the tests, since everything is done via params and return values instead of needing to know about the magic global variables and which methods mutate them.
- Replaces most explicit use of `fileSystem` with path construction using the `child*` utility methods
- Removes explicit passing of a filesystem to the commands; we're already passing a `Directory` for the
root where the tool operates, and we should never be using a different filesystem than that directory's
filesystem, so passing it was both redundant, and a potential source of test bugs.
