don't run dart gen on JS patch code

test/dart_codegen/expect/_internal/symbol.dart

Index: test/dart_codegen/expect/_internal/symbol.dart
diff --git a/test/dart_codegen/expect/_internal/symbol.dart b/test/dart_codegen/expect/_internal/symbol.dart
index 1d5e453e842a943e65fa29cf3f378415d7cb5905..88070533200bcf815a7dbe04180d8a700162732a 100644
--- a/test/dart_codegen/expect/_internal/symbol.dart
+++ b/test/dart_codegen/expect/_internal/symbol.dart
@@ -14,7 +14,7 @@ class Symbol implements core.Symbol {
       '^(?:$operatorRE\$|$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;