[cfe/ffi] Only keep Finalizables alive after their declaration

We were adding fences around the variable declaration expressions
including the variable that was not declared. This led to verification
errors and crashes when not running the verifier.
`Verification error: Variable 'VariableDeclarationImpl(final MyFinalizable myFinalizable)' used out of scope.`

TEST=pkg/vm/testcases/transformations/ffi/regress_49075.dart

Closes: https://github.com/dart-lang/sdk/issues/49075
Change-Id: I1f07f0343d29c3efb3c63c0aa0e3f20338b5c653
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/246520
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Jens Johansen <jensj@google.com>

pkg/vm/lib/transformations/ffi/finalizable.dart

@@ -220,14 +220,15 @@ mixin FinalizableTransformer on Transformer {
 
   @override
   TreeNode visitVariableDeclaration(VariableDeclaration node) {
+    node = super.visitVariableDeclaration(node) as VariableDeclaration;
     if (_currentScope == null) {
       // Global variable.
-      return super.visitVariableDeclaration(node);
+      return node;
     }
     if (_isFinalizable(node.type)) {
       _currentScope!.addDeclaration(node);
     }
-    return super.visitVariableDeclaration(node);
+    return node;
   }
 
   @override

pkg/vm/testcases/transformations/ffi/regress_49075.dart

@@ -0,0 +1,14 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:ffi';
+
+Future<void> main(List<String> arguments) async {
+  // ignore: unused_local_variable
+  final myFinalizable = await MyFinalizable();
+}
+
+class MyFinalizable implements Finalizable {
+  MyFinalizable();
+}

pkg/vm/testcases/transformations/ffi/regress_49075.dart.expect

@@ -0,0 +1,45 @@
+library #lib /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+import "dart:ffi" as ffi;
+import "dart:_internal" as _in;
+import "dart:async" as asy;
+
+import "dart:ffi";
+
+class MyFinalizable extends core::Object implements ffi::Finalizable {
+  constructor •() → self::MyFinalizable
+    : super core::Object::•() {
+    ;
+    _in::reachabilityFence(this);
+  }
+}
+static method main(core::List<core::String> arguments) → asy::Future<void> /* futureValueType= void */ /* originally async */ {
+  final asy::_Future<void> :async_future = new asy::_Future::•<void>();
+  core::bool* :is_sync = false;
+  void :return_value;
+  (dynamic) → dynamic :async_op_then;
+  (core::Object, core::StackTrace) → dynamic :async_op_error;
+  core::int :await_jump_var = 0;
+  dynamic :await_ctx_var;
+  dynamic :saved_try_context_var0;
+  function :async_op(dynamic :result_or_exception, dynamic :stack_trace) → dynamic yielding 
+    try {
+      #L1:
+      {
+        [yield] let dynamic #t1 = asy::_awaitHelper(new self::MyFinalizable::•(), :async_op_then, :async_op_error) in null;
+        final self::MyFinalizable myFinalizable = _in::unsafeCast<self::MyFinalizable>(:result_or_exception);
+        _in::reachabilityFence(myFinalizable);
+      }
+      asy::_completeWithNoFutureOnAsyncReturn(:async_future, :return_value, :is_sync);
+      return;
+    }
+    on dynamic catch(dynamic exception, core::StackTrace stack_trace) {
+      asy::_completeOnAsyncError(:async_future, exception, stack_trace, :is_sync);
+    }
+  :async_op_then = asy::_asyncThenWrapperHelper(:async_op);
+  :async_op_error = asy::_asyncErrorWrapperHelper(:async_op);
+  :async_op(null, null){() → dynamic};
+  :is_sync = true;
+  return :async_future;
+}