[ffigen] fix: objc block trampoline invalid return type (#607)

diff --git a/pkgs/ffigen/example/objective_c/config.yaml b/pkgs/ffigen/example/objective_c/config.yaml
index b44cc69..3a76517 100644
--- a/pkgs/ffigen/example/objective_c/config.yaml
+++ b/pkgs/ffigen/example/objective_c/config.yaml

@@ -12,4 +12,4 @@
   entry-points:
     - '/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/AVFAudio.framework/Headers/AVAudioPlayer.h'
 preamble: |
-  // ignore_for_file: camel_case_types, non_constant_identifier_names, unused_element, unused_field, return_of_invalid_type, void_checks, annotate_overrides, no_leading_underscores_for_local_identifiers, library_private_types_in_public_api
+  // ignore_for_file: camel_case_types, non_constant_identifier_names, unused_element, unused_field, void_checks, annotate_overrides, no_leading_underscores_for_local_identifiers, library_private_types_in_public_api

diff --git a/pkgs/ffigen/lib/src/code_generator/objc_block.dart b/pkgs/ffigen/lib/src/code_generator/objc_block.dart
index 883acd7..ba028d6 100644
--- a/pkgs/ffigen/lib/src/code_generator/objc_block.dart
+++ b/pkgs/ffigen/lib/src/code_generator/objc_block.dart

@@ -92,8 +92,14 @@
       s.write(', ${params[i].type.getDartType(w)} ${params[i].name}');
     }
     s.write(') {\n');
-    s.write('  ${isVoid ? '' : 'return '}$closureRegistry['
-        'block.ref.target.address]!(');
+    s.write('  ${isVoid ? '' : 'return '}');
+    s.write('($closureRegistry[block.ref.target.address]');
+    s.write(' as ${returnType.getDartType(w)} Function(');
+    for (int i = 0; i < params.length; ++i) {
+      s.write('${i == 0 ? '' : ', '}${params[i].type.getDartType(w)}');
+    }
+    s.write('))');
+    s.write('(');
     for (int i = 0; i < params.length; ++i) {
       s.write('${i == 0 ? '' : ', '}${params[i].name}');
     }

diff --git a/pkgs/ffigen/test/native_objc_test/setup.dart b/pkgs/ffigen/test/native_objc_test/setup.dart
index 5eff12c..8e74d87 100644
--- a/pkgs/ffigen/test/native_objc_test/setup.dart
+++ b/pkgs/ffigen/test/native_objc_test/setup.dart

@@ -119,6 +119,8 @@
 }
 
 Future<void> main(List<String> arguments) async {
+  // Allow running this script directly from any path (or an IDE).
+  Directory.current = Platform.script.resolve('.').toFilePath();
   if (!Platform.isMacOS) {
     throw OSError('Objective C tests are only supported on MacOS');
   }