promises prevent domain propagation

Test case:

``` javascript
var domain = require('domain').create();

function asyncStuff(cb) {
  new Promise(function(resolve) {
    setImmediate(resolve);
  }).then(cb);
}

domain.run(function() {

  asyncStuff(function() {
    setImmediate(function() {
      throw new Error;
    });
  });

});

domain.on('error', function() {
  console.log('caught');
});
```
