don't run dart gen on JS patch code

test/dart_codegen/expect/async/deferred_load.dart

Index: test/dart_codegen/expect/async/deferred_load.dart
diff --git a/test/dart_codegen/expect/async/deferred_load.dart b/test/dart_codegen/expect/async/deferred_load.dart
index a6624a58ef0db0429cc94b86215046b34c2f808f..01cd1226e091537e7953bb870d8e0f11f718c7d1 100644
--- a/test/dart_codegen/expect/async/deferred_load.dart
+++ b/test/dart_codegen/expect/async/deferred_load.dart
@@ -5,9 +5,7 @@ class DeferredLibrary {
   final String libraryName;
   final String uri;
   const DeferredLibrary(this.libraryName, {this.uri});
-  Future<Null> load() {
-    throw 'DeferredLibrary not supported. ' 'please use the `import "lib.dart" deferred as lib` syntax.';
-  }
+  external Future<Null> load();
 }
 class DeferredLoadException implements Exception {
   DeferredLoadException(String this._s);