Use ValueTask shared from corelib (dotnet/corefx#26636)

Signed-off-by: dotnet-bot-corefx-mirror <dotnet-bot@microsoft.com>
Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>

src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/AsyncValueTaskMethodBuilder.cs

@@ -40,8 +40,11 @@ public static AsyncValueTaskMethodBuilder<TResult> Create() =>
         /// <param name="stateMachine">The state machine instance, passed by reference.</param>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine =>
+#if netstandard
+            _methodBuilder.Start(ref stateMachine);
+#else
             AsyncMethodBuilderCore.Start(ref stateMachine); // will provide the right ExecutionContext semantics
-
+#endif
         /// <summary>Associates the builder with the specified state machine.</summary>
         /// <param name="stateMachine">The state machine instance to associate with the builder.</param>
         public void SetStateMachine(IAsyncStateMachine stateMachine) => _methodBuilder.SetStateMachine(stateMachine);