- Add a minimal implementation of Capability.

runtime/vm/message_handler.cc

Index: runtime/vm/message_handler.cc
===================================================================
--- runtime/vm/message_handler.cc	(revision 35287)
+++ runtime/vm/message_handler.cc	(working copy)
@@ -155,6 +155,12 @@
     Message::Priority saved_priority = message->priority();
     result = HandleMessage(message);
     monitor_.Enter();
+    if (FLAG_trace_isolates) {
+      OS::Print("[.] Message handled:\n"
+                "\thandler:    %s\n"
+                "\tport:       %" Pd64 "\n",
+                name(), message->dest_port());
+    }
     if (!result) {
       // If we hit an error, we're done processing messages.
       break;