Decouple callback wrapping from registration

[piscisaureus]

Right now, making an asynchronous function that takes a callback zone-aware more or less looks like the following.

// Pseudo code
function wrappedAsyncFunc(arg1, arg2, cb) {
  cb = zone.registerCallback(cb);
  try {
    return asyncFunc(arg1, arg2, cb);
  } catch (e) {
    zone.unregisterCallback(cb);
    throw e;
  }
}

I would like to ditch the try-catch block. That's more performant and also more straightforward. We can get there by decoupling wrapping from registration with the target zone. Like this:

// Pseudo code
function wrappedAsyncFunc(arg1, arg2, cb) {
  cb = zone.createCallback(cb);
  var r = asyncFunc(arg1, arg2, cb);
  zone.registerCallback(cb);
  return r;
}