Make _FutureImpl._chain do path shortening on listeners.

Make _FutureImpl._chain do path shortening on listeners.

A chained future is one that will give exactly the same result as another
future. It is used to handle, e.g., futures returned by .then handlers.

Currently we chaing futures by simply making the future itself a listener
on the future it is chained to.

This change makes the future move all its listeners to the future it is
chained to, and forward future listeners as well. It avoids long chains of
futures depending on each other.

Effectively, chained futures form equivalence classes. This makes the
equivalence classess all be represented by the future that every other element
depends on, and it applies path shortening to make the dependency graph as
flat as possible.

R=floitsch@google.com

Committed: https://code.google.com/p/dart/source/detail?r=23807

[floitsch, 2013-06-06 13:33:45]

LGTM, but I would like to see a solution for the recursion case.

https://codereview.chromium.org/15942010/diff/1/sdk/lib/async/future_impl.dart
File sdk/lib/async/future_impl.dart (right):

https://codereview.chromium.org/15942010/diff/1/sdk/lib/async/future_impl.dar...
sdk/lib/async/future_impl.dart:353: future = _resultOrListeners =
future._chainSource;
What if there is recursion?

var completer = new Completer();
var future;
future = completer.future.then((x) => future);
completer.add(499);

[Lasse Reichstein Nielsen, 2013-06-10 06:09:41]

Recursion is a problem. The example works, because it just creates the cyclic
dependency, but if you make another then that returns future, you get
stack-overflows because it tries to follow the cyclic structure.

[Lasse Reichstein Nielsen, 2013-06-10 06:18:07]

Cyclic dependencies detected. 
Should we throw instead of ignoring it?
It's a silly error, but hard to see, since none of the futures will ever
complete.

[Lasse Reichstein Nielsen, 2013-06-10 12:13:53]

Now reporting an error in case of a cyclic dependency.
PTAL.

[floitsch, 2013-06-10 12:24:37]

LGTM.

[Lasse Reichstein Nielsen, 2013-06-10 12:27:58]

Committed patchset #3 manually as r23807 (presubmit successful).