Fix some observatory analysis issues.

runtime/observatory/tests/service/evaluate_activation_in_method_class_test.dart

Index: runtime/observatory/tests/service/evaluate_activation_in_method_class_test.dart
diff --git a/runtime/observatory/tests/service/evaluate_activation_in_method_class_test.dart b/runtime/observatory/tests/service/evaluate_activation_in_method_class_test.dart
index 6f1f72731ef51d852fd7bf641cc13c9624a83cbe..daef8d0a3d341c03f273f7b60669776e54810a12 100644
--- a/runtime/observatory/tests/service/evaluate_activation_in_method_class_test.dart
+++ b/runtime/observatory/tests/service/evaluate_activation_in_method_class_test.dart
@@ -15,6 +15,7 @@ import 'evaluate_activation_in_method_class_other.dart';
 
 var topLevel = "TestLibrary";
 
+// ignore: mixin_inherits_from_not_object

[siva, 2017/06/01 19:26:33]

Not sure what this comment here means?

[rmacnak, 2017/06/01 19:30:46]

If the analyzer produces an error here, that is a bug in the analyzer.

[devoncarew, 2017/06/01 20:11:54]

It does report an issue; I believe it is correct in this case. Super mixins are
not enabled for this project
(https://github.com/flutter/flutter/blob/master/packages/flutter/lib/analysis_...),
which they probably shouldn't be, given that dart2js doesn't support them, and
this is largely a web app project.

[rmacnak, 2017/06/05 16:33:50]

Mixing-in a class that extends not-Object is allowed under the Dart standard.
This unit test runs only on the VM, not dart2js.

 class Subclass extends Superclass with Klass {
   var _instVar = 'Subclass';
   var instVar = 'Subclass';