Add UnsupportedOSPlatform annotation to internal helper class

Author: eiriktsarpalis

src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/TaskAsyncEnumerableExtensions.ToBlockingEnumerable.cs

@@ -76,6 +76,7 @@ public ManualResetEventWithAwaiterSupport()
                 _onCompleted = Set;
             }
 
+            [UnsupportedOSPlatform("browser")]
             public void Wait<TAwaiter>(TAwaiter awaiter) where TAwaiter : ICriticalNotifyCompletion
             {
                 awaiter.UnsafeOnCompleted(_onCompleted);