Write .err file alongside .txt if no .js

test/codegen_test.dart

Index: test/codegen_test.dart
diff --git a/test/codegen_test.dart b/test/codegen_test.dart
index 440a3fcaf0cc74b65f63aff7a4355f032416f33c..52dd5858b74c46873195821dbfdb75c3f88b7c09 100644
--- a/test/codegen_test.dart
+++ b/test/codegen_test.dart
@@ -121,8 +121,9 @@ main(arguments) {
 void _writeModule(String outPath, JSModuleFile result) {
   new Directory(path.dirname(outPath)).createSync(recursive: true);
 
-  result.errors.add(''); // for trailing newline
-  new File(outPath + '.txt').writeAsStringSync(result.errors.join('\n'));
+  String errors = result.errors.join('\n');
+  if (errors.isNotEmpty && !errors.endsWith('\n')) errors += '\n';
+  new File(outPath + '.txt').writeAsStringSync(errors);
 
   if (result.isValid) {
     new File(outPath + '.js').writeAsStringSync(result.code);
@@ -131,6 +132,9 @@ void _writeModule(String outPath, JSModuleFile result) {
       new File(mapPath)
           .writeAsStringSync(JSON.encode(result.placeSourceMap(mapPath)));
     }
+  } else {
+    // Also write the errors to a '.err' file for easy counting.
+    new File(outPath + '.err').writeAsStringSync(errors);
   }
 }