Instead of using package:package_config to write the .dart_tool/package_config the original json is modified and rewritten.
The .dart_tool/package_config.json file is read twice to simplify control flow.
This also avoids the issue of package:package_config writing directly to local filesystem.
Integration tests must only go through the real file system/process manager/platform. The global indirection makes this code harder to understand than if it directly referred to the concrete instances that are being used.
Update the integration shard to use a const instance of a LocalFIleSystem, LocalProcessManager, and LocalPlatform. Remove global usage and apply testWithoutContext.
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
