Infer Null for return type of functions with empty returns.

pkg/analyzer/lib/src/summary/summary_sdk.dart

 // Copyright (c) 2015, 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.summary.summary_sdk;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/file_system/file_system.dart' show ResourceProvider;
 import 'package:analyzer/src/context/context.dart';
@@ skipping 105 matching lines @@
 class SummaryTypeProvider extends TypeProviderBase {
   LibraryElement _coreLibrary;
   LibraryElement _asyncLibrary;
 
   InterfaceType _boolType;
   InterfaceType _deprecatedType;
   InterfaceType _doubleType;
   InterfaceType _functionType;
   InterfaceType _futureDynamicType;
   InterfaceType _futureNullType;
+  InterfaceType _futureOrNullType;
   InterfaceType _futureOrType;
   InterfaceType _futureType;
   InterfaceType _intType;
   InterfaceType _iterableDynamicType;
   InterfaceType _iterableType;
   InterfaceType _listType;
   InterfaceType _mapType;
   DartObjectImpl _nullObject;
   InterfaceType _nullType;
   InterfaceType _numType;
@@ skipping 47 matching lines @@
   }
 
   @override
   InterfaceType get futureNullType {
     assert(_asyncLibrary != null);
     _futureNullType ??= futureType.instantiate(<DartType>[nullType]);
     return _futureNullType;
   }
 
   @override
+  InterfaceType get futureOrNullType {
+    assert(_asyncLibrary != null);
+    _futureOrNullType ??= futureOrType.instantiate(<DartType>[nullType]);
+    return _futureOrNullType;
+  }
+
+  @override
   InterfaceType get futureOrType {
     assert(_asyncLibrary != null);
     try {
       _futureOrType ??= _getType(_asyncLibrary, "FutureOr");
     } on StateError {
       // 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 =
           TypeProviderImpl.createPlaceholderFutureOr(futureType, objectType);
     }
@@ skipping 139 matching lines @@
    * throw a [StateError] if there is no class with the given name.
    */
   InterfaceType _getType(LibraryElement library, String name) {
     Element element = library.getType(name);
     if (element == null) {
       throw new StateError("No definition of type $name");
     }
     return (element as ClassElement).type;
   }
 }