Prevent null reference when SDK lacks FutureOr<T>.

pkg/analyzer/lib/src/generated/resolver.dart

Index: pkg/analyzer/lib/src/generated/resolver.dart
diff --git a/pkg/analyzer/lib/src/generated/resolver.dart b/pkg/analyzer/lib/src/generated/resolver.dart
index 8b6fbf711ef55de721391a5ed9a0d6d059222b0f..291f075875960f92c84763b717ef6dde4a47103c 100644
--- a/pkg/analyzer/lib/src/generated/resolver.dart
+++ b/pkg/analyzer/lib/src/generated/resolver.dart
@@ -9509,12 +9509,12 @@ class TypeProviderImpl extends TypeProviderBase {
     _undefinedType = UndefinedTypeImpl.instance;
     _futureDynamicType = _futureType.instantiate(<DartType>[_dynamicType]);
     _futureNullType = _futureType.instantiate(<DartType>[_nullType]);
-    _futureOrNullType = _futureOrType.instantiate(<DartType>[_nullType]);
     _iterableDynamicType = _iterableType.instantiate(<DartType>[_dynamicType]);
     _streamDynamicType = _streamType.instantiate(<DartType>[_dynamicType]);
     // FutureOr<T> is still fairly new, so if we're analyzing an SDK that
     // doesn't have it yet, create an element for it.
     _futureOrType ??= createPlaceholderFutureOr(_futureType, _objectType);
+    _futureOrNullType = _futureOrType.instantiate(<DartType>[_nullType]);
   }
 
   /**