We'd like to support adding new assets in link hooks (#146263).

Use cases:

• The build hook outputting a static library and then the link hook making a dynamic library of it.
• Combining multiple assets into one.

Because the link hooks are run after kernel compilation, that will require using kernel concatenation instead of passing in the native asset mapping before kernel compilation.

Dart tracking issue:

• [native_assets] The native assets mapping should use kernel concatenation in AOT dart-lang/sdk#55377