Add an option to trace the debug protocol to the dart binary:

runtime/bin/main.cc

Index: runtime/bin/main.cc
===================================================================
--- runtime/bin/main.cc	(revision 29474)
+++ runtime/bin/main.cc	(working copy)
@@ -225,6 +225,16 @@
   return true;
 }
 
+bool trace_debug_protocol = false;
+static bool ProcessTraceDebugProtocolOption(const char* arg) {
+  if (*arg != '\0') {
+    return false;
+  }
+  trace_debug_protocol = true;
+  return true;
+}
+
+
 static struct {
   const char* option_name;
   bool (*process)(const char* option);
@@ -244,6 +254,7 @@
   { "--print-script", ProcessPrintScriptOption },
   { "--check-function-fingerprints", ProcessFingerprintedFunctions },
   { "--enable-vm-service", ProcessEnableVmServiceOption },
+  { "--trace-debug-protocol", ProcessTraceDebugProtocolOption },
   { NULL, NULL }
 };