Move implementation from IterableBase to Iterable.

Move implementation from IterableBase to Iterable.

Keep IterableBase around, but just extend Iterable.

There is no good reason to have IterableBase different from Iterable. The only argument is to be consistent with ListBase, but that's also inconsistent with, e.g., Stream.
ListBase only exists because List uses the default List constructor, so it's not a good super-class. As a usability-tradeoff, the ListBase class exists as a super-class.
Iterable doesn't have the same problem, so applying that "trade-off" has no up-side.

(A better long-term solution would be to be able to designate another constructor as the default "super()" constructor to call, so List could be used as a superclass).

R=sgjesse@google.com

Committed: https://code.google.com/p/dart/source/detail?r=45096

[Lasse Reichstein Nielsen, 2015-03-23 11:46:47]

lrn@google.com changed reviewers:
+ floitsch@google.com, sgjesse@google.com

[Lasse Reichstein Nielsen, 2015-04-08 09:10:53]

ping

[Søren Gjesse, 2015-04-08 09:44:58]

lgtm

[Lasse Reichstein Nielsen, 2015-04-13 12:26:37]

Committed patchset #3 (id:40001) manually as 45096 (presubmit successful).