don't run dart gen on JS patch code

test/dart_codegen/expect/async/timer.dart

Index: test/dart_codegen/expect/async/timer.dart
diff --git a/test/dart_codegen/expect/async/timer.dart b/test/dart_codegen/expect/async/timer.dart
index c0bd9c9726bbc026ddb186ccd40d774eb2725b13..808f40fc14b0e39ffe15fdce3c2315f67c7153f6 100644
--- a/test/dart_codegen/expect/async/timer.dart
+++ b/test/dart_codegen/expect/async/timer.dart
@@ -27,21 +27,9 @@ abstract class Timer {
   }
   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);