Infer Null for return type of functions with empty returns.

pkg/analyzer/test/utils.dart

 // Copyright (c) 2013, 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.test.utils;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
@@ skipping 211 matching lines @@
    * Primitive assertion for the list type
    */
   Asserter<DartType> get isList => hasElementOf(_typeProvider.listType);
 
   /**
    * Primitive assertion for the map type
    */
   Asserter<DartType> get isMap => hasElementOf(_typeProvider.mapType);
 
   /**
+   * Primitive assertion for the Null type
+   */
+  Asserter<DartType> get isNull => isType(_typeProvider.nullType);
+
+  /**
    * Primitive assertion for the num type
    */
   Asserter<DartType> get isNum => isType(_typeProvider.numType);
 
   /**
    * Primitive assertion for the Object type
    */
   Asserter<DartType> get isObject => isType(_typeProvider.objectType);
 
   /**
@@ skipping 56 matching lines @@
           Asserter<DartType> argAssert0, Asserter<DartType> argAssert1) =>
       isInstantiationOf(isMap)([argAssert0, argAssert1]);
 
   /**
    * Assert that a type is equal to the [expected].
    */
   Asserter<DartType> isType(DartType expected) => (DartType t) {
         expect(t, expected);
       };
 }