[ffigen] `symbol-address` with `@Native`

[dcharkes]

Now that we landed dart-lang/sdk@a44a524, we can start supporting symbol-address for functions.

And once dart-lang/sdk#50551 is addressed, we can add fields and the addresses of fields.

[parlough]

You and @simolus3 are both going full speed ahead with these ffi enhancements 💙