destinationReachedListener is lingering and causes expressions to combine when they shouldn't

[phet-steele]

To pull this off, you need two expressions that are the correct answer to a target on the Game screen.

1. Create two expressions that will work as an answer for a single target. This is easy in level 6, for example, where some challenges have three identical targets.
2. Correctly answer a target with one of your expressions.
3. Click Undo on that answer.
4. Take your second expression and combine it with the first expression. However, before the expressions can combine, drag the first expression into the collection area to answer the target.
5. The expressions will fail to combine, yay!
6. Press Undo again. This will position the first expression snug against the second.
7. Now just click the second expression. Or drag it around. Doesn't matter. Just make sure that when it is released, that you are releasing it over an empty collection area for which it is the correct answer (even if the collection area is really far away).

This will simultaneously answer the target (since the second expression is the correct answer) and combine the two expressions. You can freely drag this expression away from the collection area, if you'd like.

Repeat, profit:

It's theoretically possible to pull this off with single terms, but is pretty humanly difficult. Seen on macOS 10.12.5 Chrome. For phetsims/tasks/issues/860.

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[jbphet]

There was a path through the code that was not removing a listener for the end of the expression animation, and since that was lingering, the expressions could still combine in some circumstances. Nice catch @phet-steele. I have fixed this, please retest and see if the problem appears to be addressed. I was unable to duplicate it after the change, but I only tried for a few minutes.

[phet-steele]

@jbphet I can't reproduce either. Supposedly like everything else, let's leave this open to verify in the next release.

[phet-steele]

Looks good in dev.4!