don't run dart gen on JS patch code

test/dart_codegen/expect/core/string_buffer.dart

 part of dart.core;
 
 class StringBuffer implements StringSink {
-  StringBuffer([Object content = ""]) : _contents = '$content';
-  int get length => DDC$RT.cast(_contents.length, dynamic, int, "CastGeneral",
-      """line 23, column 21 of dart:core/string_buffer.dart: """,
-      _contents.length is int, true);
+  external StringBuffer([Object content = ""]);
+  external int get length;
   bool get isEmpty => length == 0;
   bool get isNotEmpty => !isEmpty;
-  void write(Object obj) {
-    _writeString('$obj');
-  }
-  void writeCharCode(int charCode) {
-    _writeString(new String.fromCharCode(charCode));
-  }
+  external void write(Object obj);
+  external void writeCharCode(int charCode);
   void writeAll(Iterable objects, [String separator = ""]) {
     Iterator iterator = objects.iterator;
     if (!iterator.moveNext()) return;
     if (separator.isEmpty) {
       do {
         write(iterator.current);
       } while (iterator.moveNext());
     } else {
       write(iterator.current);
       while (iterator.moveNext()) {
         write(separator);
         write(iterator.current);
       }
     }
   }
   void writeln([Object obj = ""]) {
     write(obj);
     write("\n");
   }
-  void clear() {
-    _contents = "";
-  }
-  String toString() => ((__x40) => DDC$RT.cast(__x40, dynamic, String,
-      "CastGeneral", """line 73, column 24 of dart:core/string_buffer.dart: """,
-      __x40 is String, true))(Primitives.flattenString(_contents));
+  external void clear();
+  external String toString();
 }