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
Review URL: https://codereview.chromium.org//15942010
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@23807 260f80e4-7a28-3924-810f-c04153c831b5
lrn@google.com
df6b4b13f8