Add to Collection interface: hasElements or equivalent

[DartBot]

This issue was originally filed by davidm...@google.com

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The most used predicate over a collection is: does it have anything in it?

But the interface provides only "does it have nothing in it", i.e. isEmpty.

This leads to lots of use of !foo.isEmpty, which is confusing as 1) it's a double negative 2) one negation happens before the main expression and the other after.

Suggestions:

 + New method, e.g. hasElements, hasItems
 + Or supplement bool some(bool f(element)) with default some((e) => true)

Discussed here:

https://groups.google.com/a/dartlang.org/d/topic/misc/MfYkrFP84dw/discussion

[sethladd]

Removed Type-Defect label.
Added Type-Enhancement, Area-Library, Triaged labels.

[lrhn]

I think we should have hasElements on Collection. It should be a getter, and so should isEmpty.

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Added this to the M2 milestone.
Added Accepted label.

[floitschG]

Removed this from the M2 milestone.
Added this to the M3 milestone.

[anders-sandholm]

Removed this from the M3 milestone.
Added this to the M4 milestone.

[larsbak]

Removed this from the M4 milestone.
Added this to the M5 milestone.

[kasperl]

Removed this from the M5 milestone.
Added this to the 1.6 milestone.

[kasperl]

Removed this from the 1.6 milestone.
Added Oldschool-Milestone-1.6 label.

[kasperl]

Removed Oldschool-Milestone-1.6 label.