Wrap runtime.js in a function.

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 60c703db07f15646ecee87e4a5b0dd682cff9383..d2a922ea7492394dd5996411fb3fd3360a5fa267 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -269,7 +269,7 @@ function ToStringCheckErrorObject(obj) {
   if (CanBeSafelyTreatedAsAnErrorObject(obj)) {
     return %_CallFunction(obj, ErrorToString);
   } else {
-    return ToString(obj);
+    return $toString(obj);
   }
 }
 
@@ -1089,7 +1089,7 @@ function DefineError(f) {
       // object. This avoids going through getters and setters defined
       // on prototype objects.
       if (!IS_UNDEFINED(m)) {
-        %AddNamedProperty(this, 'message', ToString(m), DONT_ENUM);
+        %AddNamedProperty(this, 'message', $toString(m), DONT_ENUM);
       }
     } else {
       return new f(m);