@r targets the oldest entity instead of a random entity (when selecting entities)

Verification builds:
1.2 Release_466466

Summary:
When used on entities, the @r command is not random. Instead, it simply runs on the oldest selected entity.

Impact:
This means that @r randomization for entities essentially does not work. The impact is the same as if @r was not capable of selecting entities.

What is the impact to players/partners if we don't fix?
Lack of access to this form of randomization could result in limitations on what can be efficiently created using the platform for future/current projects. Other forms of randomization are generally much more limited, and much less efficient. However, it would not have much of an impact on current content, since it is a bug with a feature that was not used in 1.1.

Repro Steps:
1. Summon five armor stands, in a row, in a world that does not contain any other armor stands.
2. Run the command "/execute @r[type=armor_stand] ~ ~ ~ setblock ~ ~1 ~ stone" (or any other command that uses @r[type=armor_stand])
3. Run "/kill @r[type=armor_stand]" five times.

Observed Results:
The command will always run on the oldest armor stand that is currently in the world. For the first command, this is always the first armor stand. For the second command, the stands will always be killed in order of placement.

Expected Results:
The command should pick a random armor stand from the five.