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

runtime/vm/service/client.dart

Index: runtime/vm/service/client.dart
diff --git a/runtime/vm/service/client.dart b/runtime/vm/service/client.dart
index a9cbd3885e1205918a8d0441c64356a6a0f793b9..1fd4f44ad8be0fac5f27427c3b77254cbe73586a 100644
--- a/runtime/vm/service/client.dart
+++ b/runtime/vm/service/client.dart
@@ -25,12 +25,16 @@ abstract class Client {
 
   /// 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.