Use PerformPromiseThen to perform some steps once a promise is settled

This partially addresses #490 (comment).

index.bs

@@ -142,6 +142,7 @@ urlPrefix: https://tc39.github.io/ecma262/; spec: ECMA-262
         text: OrdinaryGetOwnProperty; url: sec-ordinarygetownproperty
         text: OrdinaryPreventExtensions; url: sec-ordinarypreventextensions
         text: OrdinarySetWithOwnDescriptor; url: sec-ordinarysetwithowndescriptor
+        text: PerformPromiseThen; url: sec-performpromisethen
         text: Set; url: sec-set-o-p-v-throw
         text: SetFunctionLength; url: sec-setfunctionlength
         text: SetFunctionName; url: sec-setfunctionname
@@ -7896,7 +7897,7 @@ objects.
             return <emu-val>undefined</emu-val>.
         1.  Otherwise, return the result of performing any steps that were required to be run if the promise was rejected,
             with |reason| as the rejection reason.
-    1.  Return [=!=] <a abstract-op>Call</a>({{%PromiseProto_then%}}, |promise|, «|onFulfilled|, |onRejected|»).
+    1.  Return [=!=] [$PerformPromiseThen$](|promise|, |onFulfilled|, |onRejected|).
 </div>
 
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