Documentation for builtin collection should talk about deep equality

[mraleph]

Dart's builtin collections unlike collections in other languages are not compared deeply for equality - instead you need to use helpers from package:collection (e.g. ListEquality).

This fact is not in any way mentioned in the documentation for builtin classes like Set and List making it hard to discover.

I think documentation should explicitly contain the following:

• mention the fact that operator== on collections is not comparing collections contents.
• mention the fact that package:collection contains relevant helpers;
• provide explicit rationale for why default behavior of operator== was chosen - because it is somewhat counterintuitive.

/cc @leafpetersen @lrhn @eernstg

[passsy]

Relates #17963, #2217