Fix build - create output dir if needed

test/codegen_test.dart

Index: test/codegen_test.dart
diff --git a/test/codegen_test.dart b/test/codegen_test.dart
index 4b2b24db3bde4ad279ed1fcd144ca372f40a37de..dac69944d4637a86e388b74d642fcd37671ccba7 100644
--- a/test/codegen_test.dart
+++ b/test/codegen_test.dart
@@ -92,6 +92,8 @@ main(arguments) {
       var message = '''
 // Messages from compiling ${path.basenameWithoutExtension(url)}.dart
 $compilerMessages''';
+      var dir = file.parent;
+      if (!dir.existsSync()) dir.createSync(recursive: true);
       file.writeAsStringSync(message);
       compilerMessages.clear();
     });