Enable capturing of console.log output

[deva666]

Currently, all console.log calls are relayed to the Dart print function. Most of the time this is fine, but there are some use cases where we need to do something with console.log output.

[yeliulee]

temporary solution, works on flutter_js: 0.8.0

final runtime = getJavascriptRuntime();

dynamic logFn(dynamic args) {
  // do what you want
}


// register channel callback
final channelCallbacks = JavascriptRuntime.channelFunctionsRegistered[runtime.getEngineInstanceId()];
channelCallbacks!["ConsoleLog"] = logFn;

Hope this helps.