don't run dart gen on JS patch code

test/dart_codegen/expect/core/int.dart

 part of dart.core;
 
 abstract class int extends num {
-  factory int.fromEnvironment(String name, {int defaultValue}) {
-    throw new UnsupportedError(
-        'int.fromEnvironment can only be used as a const constructor');
-  }
+  external const factory int.fromEnvironment(String name, {int defaultValue});
   int operator &(int other);
   int operator |(int other);
   int operator ^(int other);
   int operator ~();
   int operator <<(int shiftAmount);
   int operator >>(int shiftAmount);
   bool get isEven;
   bool get isOdd;
   int get bitLength;
   int toUnsigned(int width);
   int toSigned(int width);
   int operator -();
   int abs();
   int get sign;
   int round();
   int floor();
   int ceil();
   int truncate();
   double roundToDouble();
   double floorToDouble();
   double ceilToDouble();
   double truncateToDouble();
   String toString();
   String toRadixString(int radix);
-  static int parse(String source, {int radix, int onError(String source)}) {
-    return ((__x24) => DDC$RT.cast(__x24, dynamic, int, "CastGeneral",
-        """line 281, column 12 of dart:core/int.dart: """, __x24 is int,
-        true))(Primitives.parseInt(source, radix, onError));
-  }
+  external static int parse(String source,
+      {int radix, int onError(String source)});
 }