don't run dart gen on JS patch code

test/dart_codegen/expect/_internal/symbol.dart

 part of dart._internal;
 
 class Symbol implements core.Symbol {
   final String _name;
   static const String reservedWordRE =
       r'(?:assert|break|c(?:a(?:se|tch)|lass|on(?:st|tinue))|d(?:efault|o)|' r'e(?:lse|num|xtends)|f(?:alse|inal(?:ly)?|or)|i[fns]|n(?:ew|ull)|' r'ret(?:hrow|urn)|s(?:uper|witch)|t(?:h(?:is|row)|r(?:ue|y))|' r'v(?:ar|oid)|w(?:hile|ith))';
   static const String publicIdentifierRE =
       r'(?!' '$reservedWordRE' r'\b(?!\$))[a-zA-Z$][\w$]*';
   static const String identifierRE =
       r'(?!' '$reservedWordRE' r'\b(?!\$))[a-zA-Z$_][\w$]*';
   static const String operatorRE =
       r'(?:[\-+*/%&|^]|\[\]=?|==|~/?|<[<=]?|>[>=]?|unary-)';
   static final RegExp publicSymbolPattern = new RegExp(
       '^(?:$operatorRE\$|$publicIdentifierRE(?:=?\$|[.](?!\$)))+?\$');
   static final RegExp symbolPattern =
       new RegExp('^(?:$operatorRE\$|$identifierRE(?:=?\$|[.](?!\$)))+?\$');
-  const Symbol(String name) : this._name = name;
+  external const Symbol(String name);
   const Symbol.unvalidated(this._name);
   Symbol.validated(String name) : this._name = validatePublicSymbol(name);
   bool operator ==(other) => other is Symbol && _name == other._name;
   int get hashCode {
     const arbitraryPrime = 664597;
     return 0x1fffffff & (arbitraryPrime * _name.hashCode);
   }
   toString() => 'Symbol("$_name")';
   static String getName(Symbol symbol) => symbol._name;
   static String validatePublicSymbol(String name) {
     if (name.isEmpty || publicSymbolPattern.hasMatch(name)) return name;
     if (name.startsWith('_')) {
       throw new ArgumentError('"$name" is a private identifier');
     }
     throw new ArgumentError('"$name" is not a valid (qualified) symbol name');
   }
   static bool isValidSymbol(String name) {
     return (name.isEmpty || symbolPattern.hasMatch(name));
   }
 }