Normative: Add Array.fromAsync

From [proposal-array-from-async](https://github.com/tc39/proposal-array-from-async).

Author: js-choi

spec.html

@@ -7393,6 +7393,20 @@ <h1>
       </emu-alg>
     </emu-clause>
 
+    <emu-clause id="sec-ifabruptcloseasynciterator" aoid="IfAbruptCloseAsyncIterator">
+      <h1>IfAbruptCloseAsyncIterator ( _value_, _iteratorRecord_ )</h1>
+      <p>IfAbruptCloseAsyncIterator is a shorthand for a sequence of algorithm steps that use an Iterator Record. An algorithm step of the form:</p>
+      <emu-alg>
+        1. IfAbruptCloseAsyncIterator(_value_, _iteratorRecord_).
+      </emu-alg>
+      <p>means the same thing as:</p>
+      <emu-alg>
+        1. Assert: _value_ is a Completion Record.
+        1. If _value_ is an abrupt completion, return ? AsyncIteratorClose(_iteratorRecord_, _value_).
+        1. Else, set _value_ to ! _value_.
+      </emu-alg>
+    </emu-clause>
+
     <emu-clause id="sec-createiterresultobject" type="abstract operation">
       <h1>
         CreateIteratorResultObject (
@@ -39843,6 +39857,82 @@ <h1>Array.from ( _items_ [ , _mapper_ [ , _thisArg_ ] ] )</h1>
         </emu-note>
       </emu-clause>
 
+      <emu-clause id="sec-array.fromAsync">
+        <h1>Array.fromAsync ( _asyncItems_ [ , _mapper_ [ , _thisArg_ ] ] )</h1>
+
+        <p>This async method performs the following steps when called:</p>
+        <emu-alg>
+          1. Let _C_ be the *this* value.
+          1. If _mapper_ is *undefined*, then
+            1. Let _mapping_ be *false*.
+          1. Else,
+            1. If IsCallable(_mapper_) is *false*, throw a *TypeError* exception.
+            1. Let _mapping_ be *true*.
+          1. Let _usingAsyncIterator_ be ? GetMethod(_asyncItems_, %Symbol.asyncIterator%).
+          1. If _usingAsyncIterator_ is *undefined*, then
+            1. Let _usingSyncIterator_ be ? GetMethod(_asyncItems_, %Symbol.iterator%).
+          1. Let _iteratorRecord_ be *undefined*.
+          1. If _usingAsyncIterator_ is not *undefined*, then
+            1. Set _iteratorRecord_ to ? GetIteratorFromMethod(_asyncItems_, _usingAsyncIterator_).
+          1. Else if _usingSyncIterator_ is not *undefined*, then
+            1. Set _iteratorRecord_ to CreateAsyncFromSyncIterator(? GetIteratorFromMethod(_asyncItems_, _usingSyncIterator_)).
+          1. If _iteratorRecord_ is not *undefined*, then
+            1. If IsConstructor(_C_) is *true*, then
+              1. Let _A_ be ? Construct(_C_).
+            1. Else,
+              1. Let _A_ be ! ArrayCreate(0).
+            1. Let _k_ be 0.
+            1. Repeat,
+              1. If _k_ ≥ 2<sup>53</sup> - 1, then
+                1. Let _error_ be ThrowCompletion(a newly created *TypeError* object).
+                1. Return ? AsyncIteratorClose(_iteratorRecord_, _error_).
+              1. Let _Pk_ be ! ToString(𝔽(_k_)).
+              1. Let _nextResult_ be ? Call(_iteratorRecord_.[[NextMethod]], _iteratorRecord_.[[Iterator]]).
+              1. Set _nextResult_ to ? Await(_nextResult_).
+              1. If _nextResult_ is not an Object, throw a *TypeError* exception.
+              1. Let _done_ be ? IteratorComplete(_nextResult_).
+              1. If _done_ is *true*, then
+                1. Perform ? Set(_A_, *"length"*, 𝔽(_k_), *true*).
+                1. Return _A_.
+              1. Let _nextValue_ be ? IteratorValue(_nextResult_).
+              1. If _mapping_ is *true*, then
+                1. Let _mappedValue_ be Completion(Call(_mapper_, _thisArg_, « _nextValue_, 𝔽(_k_) »)).
+                1. IfAbruptCloseAsyncIterator(_mappedValue_, _iteratorRecord_).
+                1. Set _mappedValue_ to Completion(Await(_mappedValue_)).
+                1. IfAbruptCloseAsyncIterator(_mappedValue_, _iteratorRecord_).
+              1. Else,
+                1. Let _mappedValue_ be _nextValue_.
+              1. Let _defineStatus_ be Completion(CreateDataPropertyOrThrow(_A_, _Pk_, _mappedValue_)).
+              1. IfAbruptCloseAsyncIterator(_defineStatus_, _iteratorRecord_).
+              1. Set _k_ to _k_ + 1.
+          1. Else,
+            1. NOTE: _asyncItems_ is neither an AsyncIterable nor an Iterable so assume it is an array-like object.
+            1. Let _arrayLike_ be ! ToObject(_asyncItems_).
+            1. Let _len_ be ? LengthOfArrayLike(_arrayLike_).
+            1. If IsConstructor(_C_) is *true*, then
+              1. Let _A_ be ? Construct(_C_, « 𝔽(_len_) »).
+            1. Else,
+              1. Let _A_ be ? ArrayCreate(_len_).
+            1. Let _k_ be 0.
+            1. Repeat, while _k_ &lt; _len_,
+              1. Let _Pk_ be ! ToString(𝔽(_k_)).
+              1. Let _kValue_ be ? Get(_arrayLike_, _Pk_).
+              1. Set _kValue_ to ? Await(_kValue_).
+              1. If _mapping_ is *true*, then
+                1. Let _mappedValue_ be ? Call(_mapper_, _thisArg_, « _kValue_, 𝔽(_k_) »).
+                1. Set _mappedValue_ to ? Await(_mappedValue_).
+              1. Else,
+                1. Let _mappedValue_ be _kValue_.
+              1. Perform ? CreateDataPropertyOrThrow(_A_, _Pk_, _mappedValue_).
+              1. Set _k_ to _k_ + 1.
+            1. Perform ? Set(_A_, *"length"*, 𝔽(_len_), *true*).
+            1. Return _A_.
+        </emu-alg>
+        <emu-note>
+          <p>This method is an intentionally generic factory method; it does not require that its *this* value be the Array constructor. Therefore it can be transferred to or inherited by any other constructors that may be called with a single numeric argument.</p>
+        </emu-note>
+      </emu-clause>
+
       <emu-clause id="sec-array.isarray">
         <h1>Array.isArray ( _arg_ )</h1>
         <p>This function performs the following steps when called:</p>