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

runtime/vm/service/client.dart

 // Copyright (c) 2013, 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.
 
 part of vmservice;
 
 // A service client.
 abstract class Client {
   /// A port for receipt of asynchronous service events.
   final RawReceivePort eventPort = new RawReceivePort();
   final VMService service;
 
   Client(this.service) {
     eventPort.handler = (response) {
       post(null, response);
     };
     service._addClient(this);
   }
 
   /// When implementing, call [close] when the network connection closes.
   void close() {
     eventPort.close();
     service._removeClient(this);
   }
 
   /// Call to process a message. Response will be posted with 'seq'.
   void onMessage(var seq, Message message) {
-    // Call post when the response arrives.
-    message.response.then((response) {
-      post(seq, response);
-    });
-    // Send message to service.
-    service.route(message);
+    try {
+      // Send message to service.
+      service.route(message).then((response) {
+        // Call post when the response arrives.
+        post(seq, response);
+      });
+    } catch (e, st) {
+      message.setErrorResponse('Internal error: $e');
+      post(seq, message.response);
+    }
   }
 
   /// When implementing, responsible for sending [response] to the client.
   void post(var seq, dynamic response);
 
   dynamic toJson() {
     return {
     };
   }
 }