Revert of Disable non-standard Promise functions in staging

test/mjsunit/es6/regress/regress-cr372788.js

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-// Flags: --allow-natives-syntax --promise-extra
+// Flags: --allow-natives-syntax
 
 var x = 0;
 var y = 0;
 
 var thenable = { then: function(f) { x++; f(); } };
 
 for (var i = 0; i < 3; ++i) {
   Promise.resolve(thenable).then(function() { x++; y++; });
 }
 assertEquals(0, x);
 
 (function check() {
   Promise.resolve().chain(function() {
     // Delay check until all handlers have run.
     if (y < 3) check(); else assertEquals(6, x);
   }).catch(function(e) { %AbortJS("FAILURE: " + e) });
 })();