davidkaya and safern Extensions for immutable collection builders (#21055) (#31071)
* Extensions for immutable builders (#21055)

Added extensions for immutable builders, which should be prefered over the extensions on IEnumerable because of performance benefits.

* Extension for immutable sorted dictionary builder (#21055)

* Extensions for immutable builders in the reference api (#21055)

* Tests for immutable builder extensions (#21055)

* Null check in extensions for immutable collection builders (#21055)
Latest commit 9f404b4 Oct 2, 2018