[stable] fix: forward --build-name and --build-number to desktop version.json - #191602

…lutter#190130)

On desktop, `flutter build linux --build-name=4.5.6` produces a
`flutter_assets/version.json` that still shows the pubspec default
(`1.0.0`) instead of the value passed on the command line. The same
happens with `--build-number`. That `version.json` (read by
`package_info_plus`) is generated by the assemble target's
`getVersionInfo`, which already honors the `BuildName` and `BuildNumber`
defines. The break is upstream in the desktop build pipeline: those
defines never reach `flutter assemble`.
`BuildInfo.toEnvironmentConfig()`, which produces the environment map
written into the generated CMake config, leaves the build name and
number out, and `tool_backend.dart` does not forward them when it
invokes assemble. Android and web are unaffected because they go through
`toBuildSystemEnvironment()`, which already includes both values.

This forwards the two values through the desktop path, following the
same pattern already used for split-debug-info and tree-shake-icons.
`toEnvironmentConfig()` now emits `BUILD_NAME` and `BUILD_NUMBER` via
the existing null-aware map entries, and `tool_backend.dart` reads them
and adds `-dBuildName` and `-dBuildNumber` to the assemble invocation
next to the other optional defines. Because the map entries are
null-aware, the keys are simply absent when the flags are not supplied,
so default behavior is unchanged. No change is needed in `linux.dart`,
which already applies these defines when they are present.

Reproduced by the issue triager and reconfirmed by another user on
3.29.2. I added a `toEnvironmentConfig` unit test in
`build_info_test.dart` that asserts the new keys appear when a build
name and number are set. The existing encoding test keeps passing
because null values stay omitted.

Fixes flutter#152236

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- [ ] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
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Co-authored-by: Ben Konyi <[email protected]>