Fix for the change/save/undo race condition.

Fix for the change/save/undo race condition.

As we found out, it is not safe to compare file modification times.
On some OSs, such as OSX with its HFS+ file modification time accuracy is just 1 second,
and it is quite possible that a file is modified more than once in 1 second.


R=brianwilkerson@google.com, paulberry@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=44080

[Paul Berry, 2015-02-26 22:23:12]

lgtm

[Brian Wilkerson, 2015-02-26 22:29:57]

LGTM

https://codereview.chromium.org/961823004/diff/1/pkg/analyzer/lib/src/util/lr...
File pkg/analyzer/lib/src/util/lru_map.dart (right):

https://codereview.chromium.org/961823004/diff/1/pkg/analyzer/lib/src/util/lr...
pkg/analyzer/lib/src/util/lru_map.dart:21: //  final LinkedHashSet<K> _lastKeys
= new LinkedHashSet<K>();
Remove this?

[scheglov, 2015-02-26 22:32:58]

Committed patchset #1 (id:1) manually as 44080 (presubmit successful).