default optional builder keys to false

lib/6to5/types/index.js

@@ -105,9 +105,15 @@ each(t.BUILDER_KEYS, function (keys, type) {
     var node = new Node;
     node.start = null;
     node.type = type;
-    each(keys, function (key, i) {
-      node[key] = args[i];
-    });
+
+    for (var i = 0; i < keys.length; i++) {
+      var arg = args[i];
+      if (arg === undefined) arg = false;
+
+      var key = keys[i];
+      node[key] = arg;
+    }
+
     return node;
   };
 });