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

runtime/vm/service/service.md

Index: runtime/vm/service/service.md
diff --git a/runtime/vm/service/service.md b/runtime/vm/service/service.md
index 571b96f2d375164a4c93d63aadee1e6bb7f1a189..5286cff9a4466d3d22b677c600004504b90caa02 100644
--- a/runtime/vm/service/service.md
+++ b/runtime/vm/service/service.md
@@ -871,6 +871,9 @@ class Context {
   // The number of variables in this context.
   int length;
 
+  // The enclosing context for this context.
+  Context parent [optional];
+
   // The variables in this context object.
   ContextElement[] variables;
 }