Improve DartLoader error handling.

sky/engine/tonic/dart_error.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "sky/engine/config.h"
 #include "sky/engine/tonic/dart_error.h"
 
 #include "base/logging.h"
 
 namespace blink {
 
 namespace DartError {
 
 const char kInvalidArgument[] = "Invalid argument.";
 
 }  // namespace DartError
 
 bool LogIfError(Dart_Handle handle) {
   if (Dart_IsError(handle)) {
     LOG(ERROR) << Dart_GetError(handle);
+
+    // Only unhandled exceptions have stacktraces.
+    if (!Dart_ErrorHasException(handle))
+      return true;
+
+    Dart_Handle stacktrace = Dart_ErrorGetStacktrace(handle);
+    const char* stacktrace_cstr = "";
+    Dart_StringToCString(Dart_ToString(stacktrace), &stacktrace_cstr);
+    LOG(ERROR) << stacktrace_cstr;
     return true;
   }
   return false;
 }
 
 }  // namespace blink