don't run dart gen on JS patch code

test/codegen_test.dart

Index: test/codegen_test.dart
diff --git a/test/codegen_test.dart b/test/codegen_test.dart
index c357983866a7c6120759f8c3f2bfb822155a1ea9..9b93a27cae6f230d3cd176b1822b6df062a9ef73 100644
--- a/test/codegen_test.dart
+++ b/test/codegen_test.dart
@@ -119,7 +119,9 @@ main(arguments) {
           forceCompile: true,
           outputDart: dartGen,
           formatOutput: dartGen);
-      var sdkPath = path.join(testDir, 'generated_sdk');
+      var sdkPath = dartGen
+          ? path.join(testDir, '..', 'tool', 'input_sdk_src')
+          : path.join(testDir, 'generated_sdk');
       var testSdk = new TypeResolver.fromDir(sdkPath, options);
       var result = compile('dart:core', testSdk, options);
       var outputDir = new Directory(path.join(actualDir, 'core'));