don't run dart gen on JS patch code

test/dart_codegen/expect/collection/hash_set.dart

 part of dart.collection;
 
 abstract class _HashSetBase<E> extends SetBase<E> {
   Set<E> difference(Set<Object> other) {
     Set<E> result = _newSet();
     for (var element in this) {
       if (!other.contains(element)) result.add(DDC$RT.cast(element, dynamic, E,
           "CastGeneral",
           """line 17, column 48 of dart:collection/hash_set.dart: """,
           element is E, false));
     }
     return result;
   }
   Set<E> intersection(Set<Object> other) {
     Set<E> result = _newSet();
     for (var element in this) {
       if (other.contains(element)) result.add(DDC$RT.cast(element, dynamic, E,
           "CastGeneral",
           """line 25, column 47 of dart:collection/hash_set.dart: """,
           element is E, false));
     }
     return result;
   }
   Set<E> _newSet();
   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);
     return result;
   }
   Iterator<E> get iterator;
 }