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

runtime/bin/dbg_message.cc

Index: runtime/bin/dbg_message.cc
diff --git a/runtime/bin/dbg_message.cc b/runtime/bin/dbg_message.cc
index 91558bf470ae674b718d047d7809d2aa4b8ac931..2a0e9056cbe8230b2aee9e99ec23455c083ac497 100644
--- a/runtime/bin/dbg_message.cc
+++ b/runtime/bin/dbg_message.cc
@@ -1097,15 +1097,17 @@ void DbgMsgQueue::MessageLoop() {
     // Handle all available vm service messages, up to a resume
     // request.
     bool resume = false;
+
+    // Release the message queue lock before handling service
+    // messages.  This allows notifications to come in while we are
+    // processing long requests and avoids deadlock with the PortMap
+    // lock in the vm.
+    msg_queue_lock_.Exit();
     while (!resume && Dart_HasServiceMessages()) {
-      // Release the message queue lock before handling service
-      // messages.  This allows notifications to come in while we are
-      // processing long requests and avoids deadlock with the PortMap
-      // lock in the vm.
-      msg_queue_lock_.Exit();
       resume = Dart_HandleServiceMessages();
-      msg_queue_lock_.Enter();
     }
+    msg_queue_lock_.Enter();
+
     if (resume) {
       break;
     }