Implement 'print', 'up', 'down', and 'frame' commands in the Observatory debugger.

runtime/vm/service_isolate.cc

Index: runtime/vm/service_isolate.cc
diff --git a/runtime/vm/service_isolate.cc b/runtime/vm/service_isolate.cc
index fd82c5a9627eb282bda580f9069775bedfabb011..650b040f1ef1d18b47ad89fcdba42a5734251c1b 100644
--- a/runtime/vm/service_isolate.cc
+++ b/runtime/vm/service_isolate.cc
@@ -219,8 +219,10 @@ class ServiceIsolateNatives : public AllStatic {
     writer.WriteMessage(message);
 
     // TODO(turnidge): Throw an exception when the return value is false?
-    PortMap::PostMessage(new Message(sp.Id(), data, writer.BytesWritten(),
-                                     Message::kOOBPriority));
+    bool result = PortMap::PostMessage(
+        new Message(sp.Id(), data, writer.BytesWritten(),
+                    Message::kOOBPriority));
+    arguments->SetReturn(Bool::Get(result));
   }
 
   static void SendRootServiceMessage(Dart_NativeArguments args) {