Address ILLink warning in InteropServices.JavaScript

In order to make the code trimming-compatible, a slight behavior change was made. The SetupJSContinuation method will only get the Result of Task<TResult> objects. If an object derives from the base Task class, and defines its own Result property, SetupJSContinuation will no longer respect that property.

This was done so the behavior remains consistent between trimmed and untrimmed applications.

Contributes to dotnet#45623

src/libraries/System.Private.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/Runtime.cs

@@ -2,10 +2,11 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Reflection;
 using System.Runtime.CompilerServices;
 using System.Threading.Tasks;
-using System.Diagnostics;
 
 namespace System.Runtime.InteropServices.JavaScript
 {
@@ -336,6 +337,9 @@ private static void SetupJSContinuation(Task task, JSObject continuationObj)
             else
                 task.GetAwaiter().OnCompleted(Complete);
 
+            [UnconditionalSuppressMessage("ReflectionAnalysis", "IL2075:UnrecognizedReflectionPattern",
+                Justification = "The DynamicDependency ensures Task<T>.Result is always preserved.")]
+            [DynamicDependency("get_Result", typeof(Task<>))]
             void Complete()
             {
                 try
@@ -348,10 +352,15 @@ void Complete()
                         {
                             result = System.Array.Empty<object>();
                         }
+                        else if (IsAssignableToGenericTaskType(task_type))
+                        {
+                            result = task_type.GetMethod("get_Result")?.Invoke(task, null);
+                        }
                         else
                         {
-                            result = task_type.GetMethod("get_Result")?.Invoke(task, System.Array.Empty<object>());
+                            result = null;
                         }
+
                         continuationObj.Invoke("resolve", result);
                     }
                     else
@@ -371,6 +380,21 @@ void Complete()
             }
         }
 
+        private static bool IsAssignableToGenericTaskType(Type? t)
+        {
+            while (t != null)
+            {
+                if (t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Task<>))
+                {
+                    return true;
+                }
+
+                t = t.BaseType;
+            }
+
+            return false;
+        }
+
         private static string ObjectToString(object o)
         {
             return o.ToString() ?? string.Empty;