There are two problems here. The first one is the tests would send
an invalid unicode character. Although we could want dicttool to
handle this more gracefully, it's fine for now.

The second problem is much more serious. If a node has more than
128 children, then the java code will crash trying to read the
dictionary back because of a bug that this change fixes. In
theory, it's possible that happens when we try to load the user
history dictionary back from the disk - native code is not affected
so there is no other point that may cause a problem.
In the practice, that means you'd need to have 129 words with a
common prefix (including empty string) but all different after
this. It's almost impossible with Google Keyboard since there are
only so many keys on the keyboard that you can make a word out
of, and then again you'd have to do it repeatedly until it
actually enters the user history dictionary, wait for it to get
saved on the disk.
The bad news is, if you manage to get this far, the keyboard will
crash every time and won't be able to get up until you clear
data for the package.
The good news is, the dictionary itself is not corrupted and only
the reading code is wrong. So updating to a newer version would
actually even recover from this situation.

All in all, considering how almost-impossible this is to trigger,
I don't think even a single user actually did hit this bug.

Bug: 8583091
Change-Id: Iabb2a7f47cbd9ed3193d2a3487318d280753e071