Intl: Use private symbols to memoize bound functions

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

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 // modification, are permitted provided that the following conditions are
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 //
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@@ skipping 19 matching lines @@
 
 // Array we want to iterate, actual numbers are not important.
 var numberArray = [1, 2, 3];
 
 // It shouldn't throw.
 // The format() method should be properly bound to the nf object.
 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);