Report HintCode.MISSING_RETURN for async functions with return type that matters.

pkg/analyzer/test/generated/non_error_resolver_test.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.test.generated.non_error_resolver_test;
 
 import 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
@@ skipping 684 matching lines @@
 import 'dart:async';
 Future<Object> f() async {
   return 5;
 }
 ''');
     await computeAnalysisResult(source);
     assertNoErrors(source);
     verify([source]);
   }
 
-  test_async_future_object_without_return() async {

[Brian Wilkerson, 2017/01/15 17:22:53]

We should test that we don't produce an error in cases where we shouldn't, such
as Future<Null>.

[scheglov, 2017/01/15 17:48:02]

There are tests:
test_async_future_null_with_return
test_async_future_null_without_return

-    Source source = addSource('''
-import 'dart:async';
-Future<Object> f() async {}
-''');
-    await computeAnalysisResult(source);
-    assertNoErrors(source);
-    verify([source]);
-  }
-
   test_async_future_with_return() async {
     Source source = addSource('''
 import 'dart:async';
 Future f() async {
   return;
 }
 ''');
     await computeAnalysisResult(source);
     assertNoErrors(source);
     verify([source]);
@@ skipping 5523 matching lines @@
   }
 
   Future<Null> _check_wrongNumberOfParametersForOperator1(String name) async {
     await _check_wrongNumberOfParametersForOperator(name, "a");
   }
 
   Future<CompilationUnit> _getResolvedLibraryUnit(Source source) async {
     return analysisContext.getResolvedCompilationUnit2(source, source);
   }
 }