Improve DartLoader error handling.

sky/engine/tonic/dart_error.cc

Index: sky/engine/tonic/dart_error.cc
diff --git a/sky/engine/tonic/dart_error.cc b/sky/engine/tonic/dart_error.cc
index 4ef6865437cb481fd3c6ed7f323fc59a5ec98e25..d733c3328a60131644add913abcf50de8fb96190 100644
--- a/sky/engine/tonic/dart_error.cc
+++ b/sky/engine/tonic/dart_error.cc
@@ -18,6 +18,15 @@ const char kInvalidArgument[] = "Invalid argument.";
 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;