Fix some observatory analysis issues.

runtime/observatory/tests/service/evaluate_activation_in_method_class_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.
 // VMOptions=--error_on_bad_type --error_on_bad_override
 
 // Tests that expressions evaluated in a frame see the same scope as the
 // frame's method.
 
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 import 'service_test_common.dart';
 
 import 'evaluate_activation_in_method_class_other.dart';
 
 var topLevel = "TestLibrary";
 
+// ignore: mixin_inherits_from_not_object
 class Subclass extends Superclass with Klass {
   var _instVar = 'Subclass';
   var instVar = 'Subclass';
   method() => 'Subclass';
   static staticMethod() => 'Subclass';
   suppress_warning() => _instVar;
 }
 
 testeeDo() {
   var obj = new Subclass();
@@ skipping 47 matching lines @@
 
   // function.Owner verus function.Origin
   // The mixin of Superclass is in _other.dart and the mixin
   // application is in _test.dart.
   result = await isolate.evalFrame(topFrame, 'topLevel');
   print(result);
   expect(result.valueAsString, equals('OtherLibrary'));
 }
 
 main(args) => runIsolateTests(args, [testerDo], testeeConcurrent: testeeDo);