Use import/export for more functions (instead of js builtins object).

src/js/messages.js

Index: src/js/messages.js
diff --git a/src/js/messages.js b/src/js/messages.js
index 5441cfe34a71970912e7b8c1fe01730b8d573b1d..387a136c6e955db44f0e345189f10e5826087269 100644
--- a/src/js/messages.js
+++ b/src/js/messages.js
@@ -4,7 +4,6 @@
 
 // -------------------------------------------------------------------
 
-var $errorToString;
 var MakeError;
 var MakeEvalError;
 var MakeRangeError;
@@ -983,8 +982,6 @@ function ErrorToString() {
 utils.InstallFunctions(GlobalError.prototype, DONT_ENUM,
                        ['toString', ErrorToString]);
 
-$errorToString = ErrorToString;
-
 MakeError = function(type, arg0, arg1, arg2) {
   return MakeGenericError(GlobalError, type, arg0, arg1, arg2);
 }
@@ -1043,4 +1040,8 @@ GlobalError.captureStackTrace = captureStackTrace;
   "uri_error_function", GlobalURIError,
 ]);
 
+utils.Export(function(to) {
+  to.ErrorToString = ErrorToString;
+});
+
 });