Revert "[vm/ffi] Roll package:ffi to Allocator and Opaque"

Revert submission 177862

Reason for revert: breaks g3 without https://github.com/flutter/engine/pull/23808
Reverted Changes:
I50b3b4c31:[vm/ffi] Roll `package:ffi` to `Allocator` and `Op...
I3f5b08c08:[vm/ffi] Change `Pointer<T extends Struct>.ref` to...
I6141c193b:[vm/ffi] Disallow empty structs

Change-Id: I128e7b096faea650e0ba0850a411eed3e97c00e6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/180185
Reviewed-by: Daco Harkes <dacoharkes@google.com>

tests/ffi_2/function_structs_test.dart

@@ -12,6 +12,7 @@ import 'dart:ffi';
 import "package:expect/expect.dart";
 import "package:ffi/ffi.dart";
 
+import 'calloc.dart';
 import 'coordinate.dart';
 import 'dylib_utils.dart';
 import 'very_large_struct.dart';