don't run dart gen on JS patch code

test/dart_codegen/expect/core/expando.dart

 part of dart.core;
 
 class Expando<T> {
   final String name;
-  Expando([String name]) : this.name = name;
+  external Expando([String name]);
   String toString() => "Expando:$name";
-  T operator [](Object object) {
-    var values = Primitives.getProperty(object, _EXPANDO_PROPERTY_NAME);
-    return ((__x21) => DDC$RT.cast(__x21, dynamic, T, "CastGeneral",
-            """line 55, column 12 of dart:core/expando.dart: """, __x21 is T,
-            false))(
-        (values == null) ? null : Primitives.getProperty(values, _getKey()));
-  }
-  void operator []=(Object object, T value) {
-    var values = Primitives.getProperty(object, _EXPANDO_PROPERTY_NAME);
-    if (values == null) {
-      values = new Object();
-      Primitives.setProperty(object, _EXPANDO_PROPERTY_NAME, values);
-    }
-    Primitives.setProperty(values, _getKey(), value);
-  }
+  external T operator [](Object object);
+  external void operator []=(Object object, T value);
 }