Call JS functions via native context instead of js builtins object.

src/proxy.js

Index: src/proxy.js
diff --git a/src/proxy.js b/src/proxy.js
index 6f18609400b793eab3356eb1fc72ce21fccfb380..b0d3d81b2839fbe6bdbc0bb177fe3dc024439b20 100644
--- a/src/proxy.js
+++ b/src/proxy.js
@@ -200,11 +200,11 @@ utils.Export(function(to) {
   to.ProxyDerivedKeysTrap = DerivedKeysTrap;
 });
 
-utils.ExportToRuntime(function(to) {
-  to["derived_get_trap"] = DerivedGetTrap;
-  to["derived_has_trap"] = DerivedHasTrap;
-  to["derived_set_trap"] = DerivedSetTrap;
-  to["proxy_enumerate"] = ProxyEnumerate;
-});
+%ExportToRuntime([
+  "derived_get_trap", DerivedGetTrap,
+  "derived_has_trap", DerivedHasTrap,
+  "derived_set_trap", DerivedSetTrap,
+  "proxy_enumerate", ProxyEnumerate,
+]);
 
 })