Infer Null for return type of functions with empty returns.

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

 // Copyright (c) 2016, 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.
 
 /**
  * This library is capable of producing linked summaries from unlinked
  * ones (or prelinked ones).  It functions by building a miniature
  * element model to represent the contents of the summaries, and then
  * scanning the element model to gather linked information and adding
  * it to the summary data structures.
@@ skipping 4711 matching lines @@
 
 class TypeProviderForLink extends TypeProviderBase {
   final Linker _linker;
 
   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;
   InterfaceType _nullType;
   InterfaceType _numType;
   InterfaceType _objectType;
@@ skipping 30 matching lines @@
 
   @override
   InterfaceType get futureDynamicType =>
       _futureDynamicType ??= futureType.instantiate(<DartType>[dynamicType]);
 
   @override
   InterfaceType get futureNullType =>
       _futureNullType ??= futureType.instantiate(<DartType>[nullType]);
 
   @override
+  InterfaceType get futureOrNullType =>
+      _futureOrNullType ??= futureOrType.instantiate(<DartType>[nullType]);
+
+  @override
   InterfaceType get futureOrType =>
       _futureOrType ??= _buildInterfaceType(_linker.asyncLibrary, 'FutureOr');
 
   @override
   InterfaceType get futureType =>
       _futureType ??= _buildInterfaceType(_linker.asyncLibrary, 'Future');
 
   @override
   InterfaceType get intType =>
       _intType ??= _buildInterfaceType(_linker.coreLibrary, 'int');
@@ skipping 240 matching lines @@
    * there are no type parameters in scope.
    */
   TypeParameterizedElementMixin get _typeParameterContext;
 
   @override
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 
   @override
   String toString() => '$enclosingElement.$name';
 }