Fix errors related to IDEA

[hastebrot]

This is my warmup PR. 😃

It adds IDEA project files to .gitignore and fixes some error messages in IDEA, so that IDEA instead shows unchecked assignment warnings.

[hastebrot]

Note that I had three assertions fails unrelated to this change. Strangely they were gone after a second test run.

[hastebrot]

The error message appeared in IDEA 14.1.4 and 142.4083.2. I first thought, this might be an EAP regression.

[TomasMikula]

Thanks for the PR. I'm not a big fan of unchecked assignments, because bugs can hide in them. Is there another way to make IDEA happy? For example, would the following make it happy:

Function<T, ObservableValue<U>> g = t -> f.apply(t);
return new FlatMappedVal<>(src, g);

Plus, to prevent someone else coming along this code and realizing the cast/extra variable is unnecessary and removing it, it should be documented that it is a workaround for a bug in IDEA compiler, with a link to their bug tracking system (so that the workaround can eventually be removed once the bug is resolved).

[TomasMikula]

Regarding the failing tests, it is a problem with the tests themselves. All those tests have to do with timing and break if scheduled events don't occur within an expected time window, typically if the system is under heavy load. I used to see those tests failing occasionally on my old laptop. The right thing to do would be to make the tests robust.