gistfile1.dart

library environment;

import "dart:async";
import "dart:io" deferred as io;
import "dart:html" deferred as html;


Future inEnvironment({Future onIo(), Future onHtml()}) {
  Completer c = new Completer();
  io.loadLibrary().then((_) {
    c.complete(onIo());
  }, onError: (_) {});
  html.loadLibrary().then((_) {
    c.complete(onHtml());
  }, onError: (_) {});
  return c.future;
}

// usage
void main() {
  inEnvironment(onIo: () {
    io.WebSocket.connect("").then((ws) {
      methodThatNeedsWebsocket(ws, ws);
    });
  }, onHtml: () {
    html.WebSocket ws = new html.WebSocket("");
    methodThatNeedsWebsocket(ws, ws);
  });
}

void methodThatNeedsWebsocket(Stream responses, StreamSink requests) {
  //[...]
}