Fixes https://github.com/flutter/flutter/issues/112833
Most of the actual changes here are in [packages/flutter_tools/lib/src/version.dart](https://github.com/flutter/flutter/pull/124558/files#diff-092e00109d9e1589fbc7c6de750e29a6ae512b2dd44e85d60028953561201605), while the rest is largely just addressing changes to the constructor of `FlutterVersion` which now has different dependencies.
This change makes `FlutterVersion` an interface with two concrete implementations:
1. `_FlutterVersionGit` which is mostly the previous implementation, and
2. `_FlutterVersionFromFile` which will read a new `.version.json` file from the root of the repo
The [`FlutterVersion` constructor](https://github.com/flutter/flutter/pull/124558/files#diff-092e00109d9e1589fbc7c6de750e29a6ae512b2dd44e85d60028953561201605R70) is now a factory that first checks if `.version.json` exists, and if so returns an instance of `_FlutterVersionFromGit` else it returns the fallback `_FlutterVersionGit` which will end up writing `.version.json` so that we don't need to re-calculate the version on the next invocation.
`.version.json` will be deleted in the bash/batch entrypoints any time we need to rebuild he tool (this will usually be because the user did `flutter upgrade` or `flutter channel`, or manually changed the commit with git).
There have been some more additional reports of a missing 'package:characters' import after upgrading flutter. This has me concerned that our pub caching logic is incorrect. Instead of the tool attempting to guess when pub should be run, always delegate to pub.
Also takes an opportunity to fix the kernel snapshot depending on the .packages or package_config. Due to the generated: date field this causes extra rebuilds. Instead when pub get is run, write out an additional file with just the package contents and version.
Fixes#66777Fixes#65723
There have been some more additional reports of a missing 'package:characters' import after upgrading flutter, as well as problems with detecting the correct language version. This has me concerned that our pub caching logic is incorrect. Instead of the tool attempting to guess when pub should be run, always delegate to pub.
Allow configuring the flutter_manifest to support a synthetic package, this is done through flutter: generate: true.
When running pub get, insert a flutter_gen entry into the packages if it does not already exist. This points to .dart_tool/flutter_gen, which can be updated to contain the generated intl sources (But doesn't currently)
Adds an integration test that verifies this code can be run and imported when enabled.
Part of #60914
