foo_foo.dart

sayHi() {
  print('hi');
}

loadme.dart

import 'package:foo/foo.dart';

main() {
  print('hello from loadme');
  sayHi();
}

start.dart

import 'dart:io';
import 'dart:isolate';

main() {
  var weAreHereUri = Platform.script.resolve(".");
  var packageConfig = "foo:${weAreHereUri.toString()}foo/";
  var dataUri = new Uri.dataFromString(packageConfig);
  print(dataUri);
  Isolate.spawnUri(new Uri.file('loadme.dart'), [], null, packageConfig: dataUri);
}