don't run dart gen on JS patch code

test/dart_codegen/expect/core/object.dart

 part of dart.core;
 
 class Object {
   const Object();
   bool operator ==(other) => identical(this, other);
-  int get hashCode => ((__x30) => DDC$RT.cast(__x30, dynamic, int,
-      "CastGeneral", """line 73, column 23 of dart:core/object.dart: """,
-      __x30 is int, true))(Primitives.objectHashCode(this));
-  String toString() => ((__x31) => DDC$RT.cast(__x31, dynamic, String,
-      "CastGeneral", """line 78, column 24 of dart:core/object.dart: """,
-      __x31 is String, true))(Primitives.objectToString(this));
-  dynamic noSuchMethod(Invocation invocation) {
-    throw new NoSuchMethodError(this, invocation.memberName,
-        invocation.positionalArguments, invocation.namedArguments);
-  }
-  Type get runtimeType => ((__x32) => DDC$RT.cast(__x32, dynamic, Type,
-      "CastGeneral", """line 101, column 27 of dart:core/object.dart: """,
-      __x32 is Type, true))(getRuntimeType(this));
+  external int get hashCode;
+  external String toString();
+  external dynamic noSuchMethod(Invocation invocation);
+  external Type get runtimeType;
 }