[builtins] Fix ToString in Array.prototype.join.

src/runtime.js

Index: src/runtime.js
diff --git a/src/runtime.js b/src/runtime.js
index f0bc13340b62f3050443afdc3e56e810d77b6637..c3b8ed22a1c4d6135d7ab9b13dc90b7f1d38de35 100644
--- a/src/runtime.js
+++ b/src/runtime.js
@@ -11,7 +11,6 @@
 
 // The following declarations are shared with other native JS files.
 // They are all declared at this one spot to avoid redeclaration errors.
-var $defaultString;
 var $NaN;
 var $nonNumberToNumber;
 var $sameValue;
@@ -345,7 +344,6 @@ function ToPositiveInteger(x, rangeErrorIndex) {
 // ----------------------------------------------------------------------------
 // Exports
 
-$defaultString = DefaultString;
 $NaN = %GetRootNaN();
 $nonNumberToNumber = NonNumberToNumber;
 $sameValue = SameValue;