Write .err file alongside .txt if no .js

test/codegen_test.dart

 // Copyright (c) 2016, 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.
 
 /// Tests code generation.
 /// Runs Dart Dev Compiler on all input in the `codegen` directory and checks
 /// that the output is what we expected.
 library dev_compiler.test.codegen_test;
 
 import 'dart:convert' show JSON;
@@ skipping 103 matching lines @@
       var generatedSdkDir =
           path.join(testDirectory, '..', 'tool', 'generated_sdk');
       return build_sdk.main(['--dart-sdk', generatedSdkDir, '-o', expectDir]);
     });
   }
 }
 
 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);
     if (result.sourceMap != null) {
       var mapPath = outPath + '.js.map';
       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);
   }
 }
 
 void _buildSunflower(ModuleCompiler compiler, String expectDir) {
   var baseDir = path.join(inputDir, 'sunflower');
   var files = ['sunflower', 'circle', 'painter']
       .map((f) => path.join(baseDir, '$f.dart'))
       .toList();
   var input = new BuildUnit('sunflower', baseDir, files, _moduleForLibrary);
   var options = new CompilerOptions(summarizeApi: false);
@@ skipping 144 matching lines @@
 /// Simplified from ParseDartTask.resolveDirective.
 String _resolveDirective(UriBasedDirective directive) {
   StringLiteral uriLiteral = directive.uri;
   String uriContent = uriLiteral.stringValue;
   if (uriContent != null) {
     uriContent = uriContent.trim();
     directive.uriContent = uriContent;
   }
   return directive.validate() == null ? uriContent : null;
 }