[pkg][vm] Native Assets builder

This package contains the logic for building native assets.

This package is the backend that invokes toplevel `build.dart` scripts.
For more info on these scripts see https://github.com/dart-lang/native.

This is a separate package so that dartdev and flutter_tools can reuse
the same logic without flutter_tools having to import dartdev.

Some design decisions:

* We don't yet have `build_dependencies`, so we use the ordinary
  dependency graph for ordering of native assets builds. (If there is
  a cycle we refuse to run.)
  Bug: https://github.com/dart-lang/pub/issues/3794
* Builds are cached based on all the configuration provided by the
  caller. Environment variables are ignored in caching. This CL also
  contains a unit test that invokes the build by not passing through
  environment variables. However, for Windows we need to pass through
  at least `SYSTEMROOT` for MSVC to run correctly. So we might need
  to further explore if we can/want to lock env variables down.
  Bug: https://github.com/dart-lang/native/issues/32
  Bug: https://github.com/dart-lang/native/issues/33

Run tests:
```
dart tools/generate_package_config.dart && \
tools/test.py -n unittest-asserts-release-linux pkg/native_assets_builder
```

Bug: https://github.com/dart-lang/sdk/issues/50565
Change-Id: I133052d7195373e87d20924d61e1e96e3d34ce8f
Cq-Include-Trybots: luci.dart.try:pkg-linux-debug-try,pkg-linux-release-try,pkg-mac-release-arm64-try,pkg-mac-release-try,pkg-win-release-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/300203
Reviewed-by: Liam Appelbe <liama@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Hossein Yousefi <yousefi@google.com>