Migrating to `dart:ffi` in Dart 2.12 and `package:ffi` 1.0.0.
Migrating to `dart:ffi` in Dart 2.12 and `package:ffi` 1.0.0.

Migrates:
- `allocate` and `free` to `calloc` and `calloc.free`.
- `Utf8.toUtf8`, `Utf8.fromUtf8`, `Utf16.toUtf16` and , `Utf16.fromUtf16` to new
  extension methods `toNativeUtf8`, `toNativeUtf16`, and `toDartString`.
- empty `Struct`s to `Opaque`.

Tested:
    TAP for global presubmit queue
    http://test/OCL:357169869:BASE:357174266:1613138261627:d1691ac7
PiperOrigin-RevId: 357175000

dart-lang/intl