Restore weak mode errors on empty returns in Null functions

pkg/analyzer/test/generated/strong_mode_test.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.
 
 library analyzer.test.generated.strong_mode_test;
 
 import 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
@@ skipping 1125 matching lines @@
     ''');
     _isFunction2Of(_isInt, _isNull)(
         invoke.argumentList.arguments[0].staticType);
     _isFutureOfDynamic(invoke.staticType);
   }
 
   test_futureOr_return_null() async {
     MethodInvocation invoke = await _testFutureOr(r'''
     FutureOr<T> mk<T>(Future<T> x) => x;
     Future<int> f;
-    test() => f.then((int x) {});
+    test() => f.then((int x) {return null;});
     ''');
     _isFunction2Of(_isInt, _isNull)(
         invoke.argumentList.arguments[0].staticType);
     _isFutureOfDynamic(invoke.staticType);
   }
 
   test_futureOr_upwards1() async {
     // Test that upwards inference correctly prefers to instantiate type
     // variables with the "smaller" solution when both are possible.
     MethodInvocation invoke = await _testFutureOr(r'''
@@ skipping 34 matching lines @@
     ''');
     _isFunction2Of(_isInt, _isNull)(
         invoke.argumentList.arguments[0].staticType);
     _isFutureOfNull(invoke.staticType);
   }
 
   test_futureOrNull_return_null() async {
     MethodInvocation invoke = await _testFutureOr(r'''
     FutureOr<T> mk<T>(Future<T> x) => x;
     Future<int> f;
-    test() => f.then<Null>((int x) {});
+    test() => f.then<Null>((int x) { return null;});
     ''');
     _isFunction2Of(_isInt, _isNull)(
         invoke.argumentList.arguments[0].staticType);
     _isFutureOfNull(invoke.staticType);
   }
 
   test_inference_hints() async {
     Source source = addSource(r'''
       void main () {
         var x = 3;
@@ skipping 2343 matching lines @@
   var v = x;
   v; // marker
 }
 int x = 3;
 ''';
     CompilationUnit unit = await resolveSource(code);
     assertPropagatedAssignedType(code, unit, typeProvider.intType, null);
     assertTypeOfMarkedExpression(code, unit, typeProvider.intType, null);
   }
 }