Fix test added by a5a8d503df69; apparently Proxy.fix is only available in debug builds (r=burning)

--HG--
extra : rebase_source : a39bd49833096b0db4baeb9b9ce5af98b3914e23

js/src/jit-test/tests/basic/testBug660734.js

@@ -1,4 +1,6 @@
-var handler = { fix: function() { return []; } };
-var p = Proxy.createFunction(handler, function(){}, function(){});
-Proxy.fix(p);
-new p();
+if (Proxy.fix) {
+    var handler = { fix: function() { return []; } };
+    var p = Proxy.createFunction(handler, function(){}, function(){});
+    Proxy.fix(p);
+    new p();
+}