don't run dart gen on JS patch code

test/dart_codegen/expect/core/regexp.dart

Index: test/dart_codegen/expect/core/regexp.dart
diff --git a/test/dart_codegen/expect/core/regexp.dart b/test/dart_codegen/expect/core/regexp.dart
index c933ec0f16b0fdd0732736a18c51cf2acdb72614..8f8a39975878b633541ad8d79ca1c75f7186d131 100644
--- a/test/dart_codegen/expect/core/regexp.dart
+++ b/test/dart_codegen/expect/core/regexp.dart
@@ -11,12 +11,8 @@ abstract class Match {
   Pattern get pattern;
 }
 abstract class RegExp implements Pattern {
-  factory RegExp(String source,
-      {bool multiLine: false, bool caseSensitive: true}) => ((__x33) => DDC$RT
-              .cast(__x33, dynamic, RegExp, "CastExact",
-                  """line 130, column 8 of dart:core/regexp.dart: """,
-                  __x33 is RegExp, true))(new JSSyntaxRegExp(source,
-          multiLine: multiLine, caseSensitive: caseSensitive));
+  external factory RegExp(String source,
+      {bool multiLine: false, bool caseSensitive: true});
   Match firstMatch(String input);
   Iterable<Match> allMatches(String input, [int start = 0]);
   bool hasMatch(String input);