fix: YieldPool unsupported code for Unity 2019 is now only available in UNITY_2020_1_OR_NEWER (#95)

Author: stan-osipov

Runtime/Async/Coroutine/Pooled YieldInstruction/WaitForSecondsPooled.cs

@@ -1,3 +1,4 @@
+#if UNITY_2020_1_OR_NEWER
 using UnityEngine;
 
 namespace StansAssets.Foundation.Async
@@ -34,3 +35,4 @@ public WaitForSecondsPooled Wait(float time)
         }
     }
 }
+#endif

Runtime/Async/Coroutine/Pooled YieldInstruction/WaitForSecondsRealtimePooled.cs

@@ -1,3 +1,4 @@
+#if UNITY_2020_1_OR_NEWER
 using UnityEngine;
 
 namespace StansAssets.Foundation.Async
@@ -34,3 +35,4 @@ public WaitForSecondsRealtimePooled Wait(float time)
         }
     }
 }
+#endif

Runtime/Async/Coroutine/Pooled YieldInstruction/YieldPool.cs

@@ -12,10 +12,13 @@ public static class YieldPool
         static YieldPool()
         {
             s_Instructions = new Dictionary<Type, ObjectPool<PooledYieldInstruction>>();
-            Add<WaitForSecondsPooled>();
+            
             Add<WaitUntilPooled>();
             Add<WaitWhilePooled>();
+#if UNITY_2020_1_OR_NEWER
+            Add<WaitForSecondsPooled>();
             Add<WaitForSecondsRealtimePooled>();
+#endif
         }
 
         static void Add<T>() where T : PooledYieldInstruction, new()
@@ -34,6 +37,7 @@ internal static void BackToPool<T>(T pooled) where T : PooledYieldInstruction
             return (T)s_Instructions[typeof(T)].Get();
         }
 
+#if UNITY_2020_1_OR_NEWER
         /// <summary>
         ///     Wait for a given number of seconds using scaled time.
         ///     <param name="seconds">Delay execution by the amount of time in seconds.</param>
@@ -51,6 +55,7 @@ public static WaitForSecondsRealtimePooled WaitForSecondsRealtime(float seconds)
         {
             return GetFromPool<WaitForSecondsRealtimePooled>().Wait(seconds);
         }
+#endif
 
         /// <summary>
         ///     Suspends the coroutine execution until the supplied delegate evaluates to false.