don't run dart gen on JS patch code

test/dart_codegen/expect/collection/hash_map.dart

 part of dart.collection;
 
 bool _defaultEquals(a, b) => a == b;
 int _defaultHashCode(a) => DDC$RT.cast(a.hashCode, dynamic, int, "CastGeneral",
     """line 10, column 28 of dart:collection/hash_map.dart: """,
     a.hashCode is int, true);
 typedef bool _Equality<K>(K a, K b);
 typedef int _Hasher<K>(K object);
 abstract class HashMap<K, V> implements Map<K, V> {
-  factory HashMap({bool equals(K key1, K key2), int hashCode(K key),
-      bool isValidKey(potentialKey)}) {
-    if (isValidKey == null) {
-      if (hashCode == null) {
-        if (equals == null) {
-          return new _HashMap<K, V>();
-        }
-        hashCode = _defaultHashCode;
-      } else {
-        if (identical(identityHashCode, hashCode) &&
-            identical(identical, equals)) {
-          return new _IdentityHashMap<K, V>();
-        }
-        if (equals == null) {
-          equals = _defaultEquals;
-        }
-      }
-    } else {
-      if (hashCode == null) {
-        hashCode = _defaultHashCode;
-      }
-      if (equals == null) {
-        equals = _defaultEquals;
-      }
-    }
-    return new _CustomHashMap<K, V>(equals, hashCode, isValidKey);
-  }
-  factory HashMap.identity() = _IdentityHashMap<K, V>;
+  external factory HashMap({bool equals(K key1, K key2), int hashCode(K key),
+      bool isValidKey(potentialKey)});
+  external factory HashMap.identity();
   factory HashMap.from(Map other) {
     HashMap<K, V> result = new HashMap<K, V>();
     other.forEach((k, v) {
       result[k] = DDC$RT.cast(v, dynamic, V, "CastGeneral",
-          """line 112, column 40 of dart:collection/hash_map.dart: """, v is V,
+          """line 87, column 40 of dart:collection/hash_map.dart: """, v is V,
           false);
     });
     return result;
   }
   factory HashMap.fromIterable(Iterable iterable,
       {K key(element), V value(element)}) {
     HashMap<K, V> map = new HashMap<K, V>();
     Maps._fillMapWithMappedIterable(map, iterable, key, value);
     return map;
   }
   factory HashMap.fromIterables(Iterable<K> keys, Iterable<V> values) {
     HashMap<K, V> map = new HashMap<K, V>();
     Maps._fillMapWithIterables(map, keys, values);
     return map;
   }
 }