This change causes a new $digest to be scheduled in the next tick if
a task was was sent to the $evalAsync queue from outside of a $digest
or an $apply.
While this mode of operation is not common for most of the user code,
this change means that $q promises that utilze $evalAsync queue to
guarantee asynchronicity of promise apis will now also resolve outside
of a $digest, which turned out to be a big pain point for some developers.
The implementation ensures that we don't do more work than needed and
that we coalese as much work as possible into a single $digest.
The use of $browser instead of setTimeout ensures that we can mock out
and control the scheduling of "auto-flush", which should in theory
allow all of the existing code and tests to work without negative
side-effects.
Closes#3539Closes#2438
Having one async queue per scope complicates the matters when users wish to do
partial scope updates, since many services put events on the rootScope. By
having single queue the programing model is simplified.
