don't run dart gen on JS patch code

test/dart_codegen/expect/collection/hash_set.dart

Index: test/dart_codegen/expect/collection/hash_set.dart
diff --git a/test/dart_codegen/expect/collection/hash_set.dart b/test/dart_codegen/expect/collection/hash_set.dart
index f9e1e1af5e1407f8f0bd4ab2149dd13b9f89aa0e..ef2fa08653b068ffaced6bca8e68235c23d7d8e3 100644
--- a/test/dart_codegen/expect/collection/hash_set.dart
+++ b/test/dart_codegen/expect/collection/hash_set.dart
@@ -25,34 +25,9 @@ abstract class _HashSetBase<E> extends SetBase<E> {
   Set<E> toSet() => _newSet()..addAll(this);
 }
 abstract class HashSet<E> implements Set<E> {
-  factory HashSet({bool equals(E e1, E e2), int hashCode(E e),
-      bool isValidKey(potentialKey)}) {
-    if (isValidKey == null) {
-      if (hashCode == null) {
-        if (equals == null) {
-          return new _HashSet<E>();
-        }
-        hashCode = _defaultHashCode;
-      } else {
-        if (identical(identityHashCode, hashCode) &&
-            identical(identical, equals)) {
-          return new _IdentityHashSet<E>();
-        }
-        if (equals == null) {
-          equals = _defaultEquals;
-        }
-      }
-    } else {
-      if (hashCode == null) {
-        hashCode = _defaultHashCode;
-      }
-      if (equals == null) {
-        equals = _defaultEquals;
-      }
-    }
-    return new _CustomHashSet<E>(equals, hashCode, isValidKey);
-  }
-  factory HashSet.identity() = _IdentityHashSet<E>;
+  external factory HashSet({bool equals(E e1, E e2), int hashCode(E e),
+      bool isValidKey(potentialKey)});
+  external factory HashSet.identity();
   factory HashSet.from(Iterable elements) {
     HashSet<E> result = new HashSet<E>();
     for (E e in elements) result.add(e);