don't run dart gen on JS patch code

test/dart_codegen/expect/collection/linked_hash_set.dart

 part of dart.collection;
 
 abstract class LinkedHashSet<E> implements HashSet<E> {
-  factory LinkedHashSet({bool equals(E e1, E e2), int hashCode(E e),
-      bool isValidKey(potentialKey)}) {
-    if (isValidKey == null) {
-      if (hashCode == null) {
-        if (equals == null) {
-          return new _LinkedHashSet<E>();
-        }
-        hashCode = _defaultHashCode;
-      } else {
-        if (identical(identityHashCode, hashCode) &&
-            identical(identical, equals)) {
-          return new _LinkedIdentityHashSet<E>();
-        }
-        if (equals == null) {
-          equals = _defaultEquals;
-        }
-      }
-    } else {
-      if (hashCode == null) {
-        hashCode = _defaultHashCode;
-      }
-      if (equals == null) {
-        equals = _defaultEquals;
-      }
-    }
-    return new _LinkedCustomHashSet<E>(equals, hashCode, isValidKey);
-  }
-  factory LinkedHashSet.identity() = _LinkedIdentityHashSet<E>;
+  external factory LinkedHashSet({bool equals(E e1, E e2), int hashCode(E e),
+      bool isValidKey(potentialKey)});
+  external factory LinkedHashSet.identity();
   factory LinkedHashSet.from(Iterable<E> elements) {
     LinkedHashSet<E> result = new LinkedHashSet<E>();
     for (final E element in elements) {
       result.add(element);
     }
     return result;
   }
   void forEach(void action(E element));
   Iterator<E> get iterator;
 }