Prevent null reference when SDK lacks FutureOr<T>.

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.generated.resolver;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
@@ skipping 9491 matching lines @@
     _numType = _getType(coreNamespace, "num");
     _objectType = _getType(coreNamespace, "Object");
     _stackTraceType = _getType(coreNamespace, "StackTrace");
     _streamType = _getType(asyncNamespace, "Stream");
     _stringType = _getType(coreNamespace, "String");
     _symbolType = _getType(coreNamespace, "Symbol");
     _typeType = _getType(coreNamespace, "Type");
     _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]);
   }
 
   /**
    * Create an [InterfaceType] that can be used for `FutureOr<T>` if the SDK
    * being analyzed does not contain its own `FutureOr<T>`.  This ensures that
    * we can analyze older SDKs.
    */
   static InterfaceType createPlaceholderFutureOr(
       InterfaceType futureType, InterfaceType objectType) {
     var compilationUnit =
@@ skipping 1371 matching lines @@
       return null;
     }
     if (identical(node.staticElement, variable)) {
       if (node.inSetterContext()) {
         result = true;
       }
     }
     return null;
   }
 }