[ffigen] Generate a nice error if files generated by test/setup.dart are not present

diff --git a/pkgs/ffigen/test/native_objc_test/native_objc_test.dart b/pkgs/ffigen/test/native_objc_test/native_objc_test.dart
index a4f2a57..8b23c2d 100644
--- a/pkgs/ffigen/test/native_objc_test/native_objc_test.dart
+++ b/pkgs/ffigen/test/native_objc_test/native_objc_test.dart

@@ -20,8 +20,9 @@
   group('native_objc_test', () {
     setUpAll(() {
       logWarnings();
-      lib = NativeObjCLibrary(DynamicLibrary.open(
-          File('test/native_objc_test/native_objc_test.dylib').absolute.path));
+      final dylib = File('test/native_objc_test/native_objc_test.dylib');
+      verifySetupFile(dylib);
+      lib = NativeObjCLibrary(DynamicLibrary.open(dylib.absolute.path));
     });
 
     test('generate_bindings', () {

diff --git a/pkgs/ffigen/test/native_test/native_test.dart b/pkgs/ffigen/test/native_test/native_test.dart
index 08e1475..d98b686 100644
--- a/pkgs/ffigen/test/native_test/native_test.dart
+++ b/pkgs/ffigen/test/native_test/native_test.dart

@@ -24,8 +24,9 @@
       } else if (Platform.isWindows) {
         dylibName = r'test\native_test\native_test.dll';
       }
-      bindings =
-          NativeLibrary(DynamicLibrary.open(File(dylibName).absolute.path));
+      final dylib = File(dylibName);
+      verifySetupFile(dylib);
+      bindings = NativeLibrary(DynamicLibrary.open(dylib.absolute.path));
     });
 
     test('generate_bindings', () {

diff --git a/pkgs/ffigen/test/test_utils.dart b/pkgs/ffigen/test/test_utils.dart
index 624e626..196da1e 100644
--- a/pkgs/ffigen/test/test_utils.dart
+++ b/pkgs/ffigen/test/test_utils.dart

@@ -31,6 +31,15 @@
   }
 }
 
+/// Check whether a file generated by test/setup.dart exists and throw a helpful
+/// exception if it does not.
+void verifySetupFile(File file) {
+  if (!file.existsSync()) {
+    throw NotFoundException("The file ${file.path} does not exist.\n\n"
+        "You may need to run: dart run test/setup.dart\n");
+  }
+}
+
 /// Generates actual file using library and tests using [expect] with expected
 ///
 /// This will not delete the actual debug file incase [expect] throws an error.