Intl: Use private symbols to memoize bound functions

test/intl/number-format/format-is-bound.js

Index: test/intl/number-format/format-is-bound.js
diff --git a/test/intl/number-format/format-is-bound.js b/test/intl/number-format/format-is-bound.js
index b24c2ed0cab91913da7ae0e1b416d521a530bcea..d93ee0067fac41c5c009fbe573bb03e77a8e4967 100644
--- a/test/intl/number-format/format-is-bound.js
+++ b/test/intl/number-format/format-is-bound.js
@@ -37,3 +37,6 @@ numberArray.forEach(nf.format);
 
 // Formatting a number should work in a direct call.
 nf.format(12345);
+
+// Reading the format doesn't add any additional property keys
+assertEquals(1, Object.getOwnPropertyNames(nf).length);