When += (and similar operations) are converted to = + ,

frog/minfrog

Index: frog/minfrog
===================================================================
--- frog/minfrog	(revision 2468)
+++ frog/minfrog	(working copy)
@@ -3574,6 +3574,9 @@
     }
     return x.invoke$4(this, name, node, new Arguments(null, [y]));
   }
+  else if ((assignKind != 0) && ((node.y instanceof BinaryExpression))) {
+    return this._visitAssign(assignKind, node.x, new ParenExpression(node.y, node.y.span), node, to$call$1(null), isVoid);
+  }
   else {
     return this._visitAssign(assignKind, node.x, node.y, node, to$call$1(null), isVoid);
   }