import from UniRx and some modified.

Assets/UniRx.Async/Internal/PromiseHelper.cs

@@ -0,0 +1,34 @@
+#if CSHARP_7_OR_LATER || (UNITY_2018_3_OR_NEWER && (NET_STANDARD_2_0 || NET_4_6))
+#pragma warning disable CS1591 // Missing XML comment for publicly visible type or member
+
+using System.Collections.Generic;
+
+namespace UniRx.Async.Internal
+{
+    internal static class PromiseHelper
+    {
+        internal static void TrySetResultAll<TPromise, T>(IEnumerable<TPromise> source, T value)
+            where TPromise : class, IResolvePromise<T>
+        {
+            var rentArray = ArrayPoolUtil.Materialize(source);
+            var clearArray = true;
+            try
+            {
+                var array = rentArray.Array;
+                var len = rentArray.Length;
+                for (int i = 0; i < len; i++) 
+                {
+                    array[i].TrySetResult(value);
+                    array[i] = null;
+                }
+                clearArray = false;
+            }
+            finally
+            {
+                rentArray.DisposeManually(clearArray);
+            }
+        }
+    }
+}
+
+#endif