Implement shareable Uri to enable rootScript use across isolates

[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 properties is Uri? rootScript

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

Line 43 in 9a86ae5

Uri? _rootScript;

.
So this issue tracks allowing Uri objects be shared, which currently is not allowed because Uri is not trivially-shareable type.