don't run dart gen on JS patch code

test/dart_codegen/expect/core/regexp.dart

 part of dart.core;
 
 abstract class Match {
   int get start;
   int get end;
   String group(int group);
   String operator [](int group);
   List<String> groups(List<int> groupIndices);
   int get groupCount;
   String get input;
   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);
   String stringMatch(String input);
   String get pattern;
   bool get isMultiLine;
   bool get isCaseSensitive;
 }