don't run dart gen on JS patch code

test/dart_codegen/expect/async/timer.dart

 part of dart.async;
 
 abstract class Timer {
   factory Timer(Duration duration, void callback()) {
     if (Zone.current == Zone.ROOT) {
       return Zone.current.createTimer(duration, callback);
     }
     return Zone.current.createTimer(
         duration, Zone.current.bindCallback(callback, runGuarded: true));
   }
   factory Timer.periodic(Duration duration, void callback(Timer timer)) {
     if (Zone.current == Zone.ROOT) {
       return Zone.current.createPeriodicTimer(duration, callback);
     }
     return Zone.current.createPeriodicTimer(duration, Zone.current
         .bindUnaryCallback(DDC$RT.wrap((void f(Timer __u126)) {
       void c(Timer x0) => f(DDC$RT.cast(x0, dynamic, Timer, "CastParam",
           """line 80, column 50 of dart:async/timer.dart: """, x0 is Timer,
           true));
       return f == null ? null : c;
     }, callback, __t129, __t127, "Wrap",
         """line 80, column 50 of dart:async/timer.dart: """,
         callback is __t127), runGuarded: true));
   }
   static void run(void callback()) {
     new Timer(Duration.ZERO, callback);
   }
   void cancel();
   bool get isActive;
-  static Timer _createTimer(Duration duration, void callback()) {
-    int milliseconds = duration.inMilliseconds;
-    if (milliseconds < 0) milliseconds = 0;
-    return ((__x131) => DDC$RT.cast(__x131, dynamic, Timer, "CastExact",
-        """line 110, column 12 of dart:async/timer.dart: """, __x131 is Timer,
-        true))(new TimerImpl(milliseconds, callback));
-  }
-  static Timer _createPeriodicTimer(
-      Duration duration, void callback(Timer timer)) {
-    int milliseconds = duration.inMilliseconds;
-    if (milliseconds < 0) milliseconds = 0;
-    return ((__x132) => DDC$RT.cast(__x132, dynamic, Timer, "CastExact",
-        """line 116, column 12 of dart:async/timer.dart: """, __x132 is Timer,
-        true))(new TimerImpl.periodic(milliseconds, callback));
-  }
+  external static Timer _createTimer(Duration duration, void callback());
+  external static Timer _createPeriodicTimer(
+      Duration duration, void callback(Timer timer));
 }
 typedef dynamic __t127(dynamic __u128);
 typedef void __t129(Timer __u130);