Finish KeyToken operations

flutter/lib/types/immediate_future.dart

@@ -2,6 +2,8 @@
 // Unfortunately Future.value(x) in FutureBuilder always results in one frame were snapshot.connectionState is waiting
 // This way we can set the ImmediateFuture.value directly and circumvent that.
 
+import 'dart:async';
+
 class ImmediateFuture<T> {
   final Future<T> future;
   final T? value;
@@ -20,6 +22,10 @@ class ImmediateFuture<T> {
       : future = Future.value(v),
         value = v;
 
+  ImmediateFuture.ofPending()
+      : future = Completer<T>().future,
+        value = null;
+
   T? get() {
     return value ?? _futureValue;
   }