Add concurrent modification check to ListMixin/IterableMixin.reduce.

Add concurrent modification check to ListMixin/IterableMixin.reduce.

Add tests of reduce and fold.

BUG= http://dartbug.com/21289
R=ajohnsen@google.com

Committed: https://code.google.com/p/dart/source/detail?r=41093

[Lasse Reichstein Nielsen, 2014-10-14 09:01:27]

lrn@google.com changed reviewers:
+ ajohnsen@google.com

[Anders Johnsen, 2014-10-14 09:08:25]

lgtm

https://codereview.chromium.org/656773002/diff/1/sdk/lib/collection/list.dart
File sdk/lib/collection/list.dart (right):

https://codereview.chromium.org/656773002/diff/1/sdk/lib/collection/list.dart...
sdk/lib/collection/list.dart:189: if (length != this.length) {
Can this be changed to only check at end?

[Lasse Reichstein Nielsen, 2014-10-14 09:12:05]

https://codereview.chromium.org/656773002/diff/1/sdk/lib/collection/list.dart
File sdk/lib/collection/list.dart (right):

https://codereview.chromium.org/656773002/diff/1/sdk/lib/collection/list.dart...
sdk/lib/collection/list.dart:189: if (length != this.length) {
Not really. We check for non-change after each call to the user function,
because it might have reduced the length so low that this[i] will throw - and to
err early, so we don't get two modifications that end up with the correct
length.

[Lasse Reichstein Nielsen, 2014-10-14 09:12:30]

Committed patchset #1 (id:1) manually as 41093 (presubmit successful).