Revert "Make EfficientLength public, as EfficientLengthIterable."

Revert "Make EfficientLength public, as EfficientLengthIterable."

It's still not a good solution for detecting an Iterable with an efficient
length. It's not inherited by, e.g., a DelegatingIterable wrapper or similar
generic Iterable transformers.

Keep this as an internal optimization for quickly detecting the most common
efficient-length Iterable classes (List, Set, Queue, Map.keys/values), but
don't make it public.

A *real* solution would be adding a hasEfficientLength getter to Iterable, or
adding an efficientLength getter that may return null if it's not efficient.
This would something that a wrapper can attach to.

R=sgjesse@google.com

Committed: https://github.com/dart-lang/sdk/commit/babcd4e9c0af02c98ae95b974e79bc6f064f16c8

[Lasse Reichstein Nielsen, 2015-05-29 09:18:35]

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

[Søren Gjesse, 2015-05-29 09:34:17]

lgtm

[Lasse Reichstein Nielsen, 2015-05-29 10:52:51]

Committed patchset #1 (id:1) manually as
babcd4e9c0af02c98ae95b974e79bc6f064f16c8 (presubmit successful).