2nd attempt at adding streamListen/streamCancel to the service protocol.

runtime/observatory/tests/service/contexts_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.
 // VMOptions=--compile-all --error_on_bad_type --error_on_bad_override
 
 library inbound_references_test;
 
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
@@ skipping 41 matching lines @@
 
 (Isolate isolate) =>
   isolate.rootLibrary.load().then((Library lib) {
     Field field = lib.variables.singleWhere((v) => v.name == 'cleanBlock');
     return field.load().then((_) {
       return field.staticValue.load().then((Instance block) {
         expect(block.isClosure, isTrue);
         expect(block.context.isContext, isTrue);
         expect(block.context.length, equals(0));
         return block.context.load().then((Context ctxt) {
-          expect(ctxt.parentContext.isNull, isTrue);
+          expect(ctxt.parentContext, isNull);
         });
       });
     });
   }),
 
 (Isolate isolate) =>
   isolate.rootLibrary.load().then((Library lib) {
     Field field = lib.variables.singleWhere((v) => v.name == 'copyingBlock');
     return field.load().then((_) {
       return field.staticValue.load().then((Instance block) {
         expect(block.isClosure, isTrue);
         expect(block.context.isContext, isTrue);
         expect(block.context.length, equals(1));
         return block.context.load().then((Context ctxt) {
           expect(ctxt.variables.single['value'].isString, isTrue);
           expect(ctxt.variables.single['value'].valueAsString, equals('I could be copied into the block'));
           expect(ctxt.parentContext.isContext, isTrue);
           expect(ctxt.parentContext.length, equals(0));
           return ctxt.parentContext.load().then((Context outerCtxt) {
-            expect(outerCtxt.parentContext.isNull, isTrue);
+            expect(outerCtxt.parentContext, isNull);
           });
         });
       });
     });
   }),
 
 (Isolate isolate) =>
   isolate.rootLibrary.load().then((Library lib) {
     Field field = lib.variables.singleWhere((v) => v.name == 'fullBlock');
     return field.load().then((_) {
       return field.staticValue.load().then((Instance block) {
         expect(block.isClosure, isTrue);
         expect(block.context.isContext, isTrue);
         expect(block.context.length, equals(1));
         return block.context.load().then((ctxt) {
           expect(ctxt.variables.single['value'].isInt, isTrue);
           expect(ctxt.variables.single['value'].valueAsString, equals('43'));
           expect(ctxt.parentContext.isContext, isTrue);
           expect(ctxt.parentContext.length, equals(0));
           return ctxt.parentContext.load().then((Context outerCtxt) {
-            expect(outerCtxt.parentContext.isNull, isTrue);
+            expect(outerCtxt.parentContext, isNull);
           });
         });
       });
     });
   }),
 
 (Isolate isolate) =>
   isolate.rootLibrary.load().then((Library lib) {
     Field field = lib.variables.singleWhere((v) => v.name == 'fullBlockWithChain');
     return field.load().then((_) {
       return field.staticValue.load().then((Instance block) {
         expect(block.isClosure, isTrue);
         expect(block.context.isContext, isTrue);
         expect(block.context.length, equals(1));
         return block.context.load().then((Context ctxt) {
           expect(ctxt.variables.single['value'].isInt, isTrue);
           expect(ctxt.variables.single['value'].valueAsString, equals('4201'));
           expect(ctxt.parentContext.isContext, isTrue);
           expect(ctxt.parentContext.length, equals(1));
           return ctxt.parentContext.load().then((Context outerCtxt) {
             expect(outerCtxt.variables.single['value'].isInt, isTrue);
             expect(outerCtxt.variables.single['value'].valueAsString, equals('421'));
             expect(outerCtxt.parentContext.isContext, isTrue);
             expect(outerCtxt.parentContext.length, equals(0));
             return outerCtxt.parentContext.load().then((Context outerCtxt2) {
-              expect(outerCtxt2.parentContext.isNull, isTrue);
+                expect(outerCtxt2.parentContext, isNull);
             });
           });
         });
       });
     });
   }),
 
 ];
 
 main(args) => runIsolateTests(args, tests, testeeBefore: script);