Delete stale .js or .err files when generating codegen output.

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 122 matching lines @@
   }
 }
 
 void _writeModule(String outPath, JSModuleFile result) {
   new Directory(path.dirname(outPath)).createSync(recursive: true);
 
   String errors = result.errors.join('\n');
   if (errors.isNotEmpty && !errors.endsWith('\n')) errors += '\n';
   new File(outPath + '.txt').writeAsStringSync(errors);
 
+  var jsFile = new File(outPath + '.js');
+  var errorFile = new File(outPath + '.err');
+
   if (result.isValid) {
-    new File(outPath + '.js').writeAsStringSync(result.code);
+    jsFile.writeAsStringSync(result.code);
     if (result.sourceMap != null) {
       var mapPath = outPath + '.js.map';
       new File(mapPath)
           .writeAsStringSync(JSON.encode(result.placeSourceMap(mapPath)));
     }
+
+    // There are no errors, so delete any stale ".err" file.
+    if (errorFile.existsSync()) {
+      errorFile.deleteSync();
+    }
   } else {
     // Also write the errors to a '.err' file for easy counting.
-    new File(outPath + '.err').writeAsStringSync(errors);
+    errorFile.writeAsStringSync(errors);
+
+    // There are errors, so delete any stale ".js" file.
+    if (jsFile.existsSync()) {
+      jsFile.deleteSync();
+    }
   }
 }
 
 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 143 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;
 }