Don't send duplicate responses for debugger commands in Sky

sky/framework/inspector/inspector.sky

 <import src="/mojo/public/sky/connection.sky" as="connection" />
 <import src="/mojo/public/sky/core.sky" as="core" />
 <import src="/mojo/public/sky/support.sky" as="support" />
 <import src="/sky/services/inspector/inspector.mojom.sky" as="inspector" />
 <import src="console-agent.sky" as="ConsoleAgent" />
 <import src="dom-agent.sky" as="DOMAgent" />
 <import src="page-agent.sky" as="PageAgent" />
 <import src="worker-agent.sky" as="WorkerAgent" />
 <import src="runtime-agent.sky" as="RuntimeAgent" />
 <import src="indexeddb-agent.sky" as="IndexedDBAgent" />
@@ skipping 11 matching lines @@
     CSS: new CSSAgent(domAgent),
     IndexedDB: new IndexedDBAgent(),
   };
   this.missingNames_ = {};
   this.unansweredMessages_ = [];
 }
 
 InspectorBackend.prototype = Object.create(
     inspector.InspectorBackend.stubClass.prototype);
 
+InspectorBackend.prototype.IMPLEMENTED_IN_CPP = "IMPLEMENTED_IN_CPP";
 InspectorBackend.prototype.ASYNC_RESPONSE = "ASYNC_RESPONSE";
 InspectorBackend.prototype.MESSAGE_TIMEOUT_MS = 30000;
 
 InspectorBackend.prototype.onConnect = function() {
 };
 
 InspectorBackend.prototype.onDisconnect = function() {
 };
 
 InspectorBackend.prototype.logMissing_ = function(name) {
   if (name in this.missingNames_)
     return;
   this.missingNames_[name] = true;
   console.log("InspectorBackend missing " + name);
 }
 
 InspectorBackend.prototype.dispatch_ = function(descriptor, params, message_id) {
   var parsed = descriptor.split('.');
 
   var agentName = parsed[0];
   var methodName = parsed[1];
 
   // Debugger is implemented in c++.
   if (agentName == "Debugger")
-    return;
+    return this.IMPLEMENTED_IN_CPP;
 
   if (!(agentName in this.agents)) {
     this.logMissing_(agentName);
     return {};
   }
 
   var agent = this.agents[agentName];
 
   if (!(methodName in agent)) {
     this.logMissing_(descriptor);
     return {};
   }
 
   try {
     return agent[methodName](params, message_id);
   } catch(ex) {
     console.log(descriptor + ": " + ex);
   }
 };
 
 InspectorBackend.prototype.onMessage = function(data) {
   var message = JSON.parse(data);
   var result = this.dispatch_(message.method, message.params, message.id);
+  if (result === this.IMPLEMENTED_IN_CPP)
+      return;
   this.unansweredMessages_.push(message.id);
   // FIXME: This magic return value is kinda hacky.
   if (result != this.ASYNC_RESPONSE)
     this.sendResponse(message.id, result);
   else {
     window.setTimeout(function() {
       if (this.unansweredMessages_.indexOf(message.id) == -1)
         return;
       console.log("Error, failed to reply to message id " + message.id);
     }.bind(this), this.MESSAGE_TIMEOUT_MS);
@@ skipping 24 matching lines @@
 
 var frontendHandle = internals.connectToService(
     "mojo:sky_inspector_server", inspector.InspectorFrontend.name);
 window.frontendConnection = new connection.Connection(
     frontendHandle,
     InspectorBackend,
     inspector.InspectorFrontend.proxyClass);
 
 window.frontend = frontendConnection.remote;
 </script>