native_add test project now lists a .h source, which breaks the Dart SDK's dart install tests with published native_toolchain_c #3743

Summary

The Dart SDK rolls this repo into its DEPS file. #3728 ("Track header
files as build dependencies") makes the Dart SDK test
pkg/dartdev/test/native_assets/install_test.dart fail. Because of that,
the latest SDK dependency roll holds dart-lang/native at 10515af, the
commit just before #3728. The other dependencies in that roll were updated
as usual.

Until this is resolved and native_rev in the SDK DEPS moves past
#3728, the SDK will not pick up new commits from this repo.

What fails

The SDK bots fail two tests in install_test.dart:

• dart install dart_app (with build hooks and code assets)
• dart install --overwrite

Both fail while building native_add:

clang --target=arm64-apple-darwin ... -fPIC -O3 ... -DRELEASE -DNDEBUG \
  .../native_add/src/native_add.c .../native_add/src/native_add.h \
  -framework Foundation --shared -o .../libnative_add.dylib ...
clang: error: cannot specify -o when generating multiple output files

Failing build (mac-arm64):
https://ci.chromium.org/b/8668117913278491681

Cause

#3728 makes two changes that only work together:

1. pkgs/hooks_runner/test_data/native_add/hook/build.dart now passes
   sources: ['src/$packageName.c', 'src/$packageName.h'].
2. CBuilder in native_toolchain_c now removes .h files from
   sources before calling the compiler. This change is only in
   0.19.6-wip. The published version is still 0.19.5.

The SDK's dart install tests copy the test_data projects from this repo.
dart install resolves dependencies in a temporary helper package, so the
dependency_overrides in the copied projects (which point at the local
checkout) are ignored. native_add depends on native_toolchain_c: any,
so dart install resolves to the published 0.19.5. That version still
passes the .h file to clang, and clang fails as shown above.

In short, the test_data changes are released together with the SDK
checkout, but the package they depend on resolves to the published
version. When a test project needs an unreleased native_toolchain_c
feature, dart install breaks.

Possible resolutions

We would like the maintainers of this repo to choose the approach. Some
options:

• Revert the .h entry in test_data/native_add (and any other test
  projects that dart install uses) until native_toolchain_c 0.19.6 is
  published.
• Publish native_toolchain_c 0.19.6, so the any constraint resolves to
  a version that filters headers.
• Keep test_data projects that the SDK tests use compatible with the
  latest published versions of this repo's packages.

The SDK tests could also be changed to point native_toolchain_c at the
local checkout, the way install_test.dart already does for hooks and
record_use. Any of these changes needs a follow-up SDK roll that moves
native_rev past #3728. The SDK side will handle that roll once a fix
lands here.

Reproduce

In a Dart SDK checkout with native_rev at 237524d or later (after
gclient sync):

python3 tools/test.py -n unittest-asserts-release-<os>-<arch> \
  pkg/dartdev/test/native_assets/install_test

Related SDK roll CL: https://dart-review.googlesource.com/c/sdk/+/563220