As far as I know, Dart was created beforehand in order to "replace" Javascript. That did never work out, it never got any real traction, so Dart eventually stumbled around without any real purpose. Then Flutter came, used Dart (because they needed some language like JS, but not JS, and Dart was "there" and could be used because it was a Google product) and then Flutter started to gain traction, and so did Dart, but only as a side effect of Flutter now being the only real "user" of the language.