Implement a way to create shareable Map

[aam]

To support dart-lang/language#4515 various core fields have to be tagged vm:shared. This is so that they are initialized once per isolate group to avoid initializing them for isolate group bound callback.

Among those fields is Map<String, Uri>? _packageMap

sdk/sdk/lib/_internal/vm/bin/builtin.dart

Line 59 in 9a86ae5

Map<String, Uri>? _packageMap = null;

So this issue tracks allowing creation of such (more-than likely immutable/const) maps, perhaps limited to dart core libraries.

[lrhn]

Consider deprecating the property instead of doing extra work to support it.

It's a kind of introspective to list the packages that are part of the program's source. It won't, or shouldn't, work with AoT compilation, because it maps names to source locations on the compilation machine.

(Nothing related to package resolution should be available in AoT build.)