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

runtime/observatory/tests/service/contexts_test.dart

Index: runtime/observatory/tests/service/contexts_test.dart
diff --git a/runtime/observatory/tests/service/contexts_test.dart b/runtime/observatory/tests/service/contexts_test.dart
index 6b3e8c9c3330c3258b06c4266d0e2acb28d83fe1..bf593f4e7610bf81cfedd8123fe3300e746e1c84 100644
--- a/runtime/observatory/tests/service/contexts_test.dart
+++ b/runtime/observatory/tests/service/contexts_test.dart
@@ -59,7 +59,7 @@ var tests = [
         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);
         });
       });
     });
@@ -79,7 +79,7 @@ var tests = [
           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);
           });
         });
       });
@@ -100,7 +100,7 @@ var tests = [
           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);
           });
         });
       });
@@ -126,7 +126,7 @@ var tests = [
             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);
             });
           });
         });