Add gdb helper functions for creating a handle and using ToCString with RawObjects.

runtime/lib/object.cc

Index: runtime/lib/object.cc
diff --git a/runtime/lib/object.cc b/runtime/lib/object.cc
index f0739adc3acdfdebdf0a6430ee4fdc9d0bbde187..340f35e0ebe6211c7081fdb3bc919c070f813d7b 100644
--- a/runtime/lib/object.cc
+++ b/runtime/lib/object.cc
@@ -17,15 +17,15 @@ namespace dart {
 
 DECLARE_FLAG(bool, trace_type_checks);
 
-// Helper function in stacktrace.cc.
-void _printCurrentStackTrace();
+// Helper function in gdb_helpers.cc.
+void _printDartStackTrace();
 
 DEFINE_NATIVE_ENTRY(DartAsync_fatal, 1) {
   // The dart:async library code entered an unrecoverable state.
   const Instance& instance = Instance::CheckedHandle(arguments->NativeArgAt(0));
   const char* msg = instance.ToCString();
   OS::PrintErr("Fatal error in dart:async: %s\n", msg);
-  _printCurrentStackTrace();
+  _printDartStackTrace();
   FATAL(msg);
   return Object::null();
 }