Add an option (possibly the default?) for DeepCollectionEquality to handle self-referential structures

[nex3]

Sometimes it's useful to be able to compare nested object structures that can contain self-referential loops. Currently DeepCollectionEquality causes a stack overflow when passed such structures, but it would be nice if it could handle them. This could be done either as a configuration option or as the default if the prospect of storing back-references isn't too worrisome.

Currently the YAML package has its own custom implementation of deep equality, but it would be nice if it could use the standard implementation instead.

[lrhn]

That should be doable. For each operand, remember each object it is currently being compared to, and if you see that combination again, return true.
It's not fool-proof, but it will catch most simple self-referential structures.

[nex3]

I believe YAML's deepEquals operation works like that, but it remembers the order of the objects it's being compared to as well.

[kevmoo]

Aligning w/ the bits offered from matcher would be nice, too.

Fewer implementations FTW

[nex3]

Matcher's use case is a little different, because it needs to track a bunch of extra information to provide useful messaging when the equality fails.

[DartBot]

This issue has been moved to dart-lang/collection#6.