Revert "[vm/ffi] Remove deprecated Struct.addressOf getter"

This reverts commit 65fab23.

Reason for revert: These can't be remove until the next dev roll of Flutter. See flutter/flutter#76129.

Original change's description:
> [vm/ffi] Remove deprecated `Struct.addressOf` getter
>
> This got deprecated in Dart 2.12 stable. Removing for the next release.
>
> Closes: #40667
>
> Change-Id: Ifdbcf76178be6fca8603d478d629b6b6655b7123
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/184463
> Reviewed-by: Clement Skau <cskau@google.com>
> Commit-Queue: Daco Harkes <dacoharkes@google.com>

Change-Id: I94c22a206f3668ddb72d284afdc3b048d8b11aaa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/185500
Commit-Queue: Zach Anderson <zra@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>

sdk/lib/ffi/struct.dart

@@ -58,3 +58,10 @@ abstract class Struct extends NativeType {
 
   Struct._fromPointer(this._addressOf);
 }
+
+/// Extension on [Struct] specialized for its subtypes.
+extension StructAddressOf<T extends Struct> on T {
+  /// Returns the address backing the reference.
+  @Deprecated('Hold on to the pointer backing a struct instead.')
+  Pointer<T> get addressOf => _addressOf as Pointer<T>;
+}

tests/ffi/structs_nnbd_workaround_test.dart

@@ -98,6 +98,8 @@ void testTypeTest() {
   final pointer = calloc<Coordinate>();
   Coordinate c = pointer.ref;
   Expect.isTrue(c is Struct);
+  // TODO(https://dartbug.com/40667): Remove support for this.
+  Expect.isTrue(c.addressOf is Pointer<Coordinate>);
   calloc.free(pointer);
 }
 

tests/ffi/structs_test.dart

@@ -128,6 +128,8 @@ void testTypeTest() {
   final pointer = calloc<Coordinate>();
   Coordinate c = pointer.ref;
   Expect.isTrue(c is Struct);
+  // TODO(https://dartbug.com/40667): Remove support for this.
+  Expect.isTrue(c.addressOf is Pointer<Coordinate>);
   calloc.free(pointer);
 }
 

tests/ffi_2/structs_test.dart

@@ -128,6 +128,8 @@ void testTypeTest() {
   final pointer = calloc<Coordinate>();
   Coordinate c = pointer.ref;
   Expect.isTrue(c is Struct);
+  // TODO(https://dartbug.com/40667): Remove support for this.
+  Expect.isTrue(c.addressOf is Pointer<Coordinate>);
   calloc.free(pointer);
 }