Harden a few builtins

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index d20b34b07e26e587ef9e9e1e03637750fb1f8e33..44ae4b8c849dcb748d44c21f986bd026489a792b 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -162,7 +162,9 @@ var kMessages = {
   symbol_to_primitive:           ["Cannot convert a Symbol wrapper object to a primitive value"],
   invalid_module_path:           ["Module does not export '", "%0", "', or export is not itself a module"],
   module_type_error:             ["Module '", "%0", "' used improperly"],
-  module_export_undefined:       ["Export '", "%0", "' is not defined in module"]
+  module_export_undefined:       ["Export '", "%0", "' is not defined in module"],
+  // Internal error.
+  internal_error:                ["Internal error"]
 };
 
 
@@ -349,6 +351,11 @@ function MakeError(type, args) {
   return MakeGenericError($Error, type, args);
 }
 
+
+function MakeInternalError() {
+  return MakeGenericError($InternalError, "internal_error");
+}
+
 /**
  * Find a line number given a specific source position.
  * @param {number} position The source position.
@@ -1184,7 +1191,7 @@ function SetUpError() {
     %SetProperty(global, name, f, DONT_ENUM);
     %SetProperty(builtins, '$' + name, f, DONT_ENUM | DONT_DELETE | READ_ONLY);
     // Configure the error function.
-    if (name == 'Error') {
+    if (name === 'Error') {
       // The prototype of the Error object must itself be an error.
       // However, it can't be an instance of the Error object because
       // it hasn't been properly configured yet.  Instead we create a
@@ -1193,7 +1200,7 @@ function SetUpError() {
       %FunctionSetPrototype(ErrorPrototype, $Object.prototype);
       %FunctionSetInstanceClassName(ErrorPrototype, 'Error');
       %FunctionSetPrototype(f, new ErrorPrototype());
-    } else {
+    } else if (name !== 'InternalError') {
       %FunctionSetPrototype(f, new $Error());
     }
     %FunctionSetInstanceClassName(f, 'Error');
@@ -1224,6 +1231,7 @@ function SetUpError() {
   DefineError(function ReferenceError() { });
   DefineError(function EvalError() { });
   DefineError(function URIError() { });
+  DefineError(function InternalError() { });
 }
 
 SetUpError();