Minor cleanup of the script to create the build-all app, mostly for Android:
- Don't override the minSdk to 21, since the Flutter-provided value is now 21 already.
- Don't enable multidex, since that isn't necessary for minSdk 20+.
- Remove camera_android_camerax from the direct dependency list, not camera_android, now that the endorsement has switched. This will return us to the previous, intended behavior of testing that both versions build together.
- If for some reason the Dart SDK isn't detected, use 3.0.0+ instead of 2.12.0+.
Adds the ability to replace portions of the `flutter create`d app with saved copies, and adds a second build-all phase for Android that uses a Flutter 2.0.6-created `android/` directory (AGP 4.1, Gradle 6.7) to catch issues like https://github.com/flutter/flutter/issues/125621 and https://github.com/flutter/flutter/issues/125482 prior to release.
Includes some incidental cleanup:
- Extracts a helper method for adjusting files, so that this doesn't add even more copies of basically identical code.
- (This was motivated by an earlier version of the PR that added modifications to several more files, which I ended up undoing, but the cleanup seemed worth keeping.)
- Adds missing unit test coverage.
- Reworks the unit tests to use a mock process manager and dummy files, instead of each test actually calling `flutter create`, which made each new test add several seconds to the unit test suite.
- While this reduces the integration-style coverage, in practice the integration tests of the repo tooling is the CI itself, so the unit tests should be true unit tests.
- Changes the non-legacy test to Kotlin; we were still testing with Java even though Kotlin has been the default for quite a while, so we weren't testing what most new users would actually be running. Since we now have a legacy test, I used Java there to cover both.
- Removes some dead code for modifying the AndroidManifest.xml; when trying to set up unit tests for it I discovered that it no longer matches anything in an actual project. It dates back to the original command, and seems to have been a camera-related hack, which we clearly no longer need since it wasn't working and camera still works in build-all.
This is captured somewhat in the README in the legacy project directory, but to document the approach here: originally I was going to add flags to change individual items (AGP version, Gradle version), but quickly ran into the fact that selective downgrading is extremely fragile. E.g.,:
- The Kotlin version set in current projects doesn't work when downgrading AGP and Gradle.
- The app template can unconditionally use anything (e.g., `namespace`) that the AGP version it uses supports, so arbitrary future breakage is possible.
It's also less useful as a real test of what a plugin client's project likely looks like. Starting with a complete platform directory, and doing whatever the minimal changes are to keep it working, will likely reflect a common real-world scenario. On the flip side, the reason this doesn't use a complete 2.0.6 project, but instead is based on specific platform directories, is that we don't want to waste time manually maintaining, e.g., old Dart code that is irrelevant to the goal of the test. For now this only uses Android because that's where we've seen problems in practice, but we can alway add other legacy platform tests later if we find a need.
Fixes https://github.com/flutter/flutter/issues/125689
