don't run dart gen on JS patch code

test/dart_codegen/expect/async/deferred_load.dart

 part of dart.async;
 
 @Deprecated("Dart sdk v. 1.8")
 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);
   String toString() => "DeferredLoadException: '$_s'";
   final String _s;
 }