Except you don't get easily-readable messages for failures that way. With this matcher, you'll get a message like "Expected _specialsCache to contain key test/spec/LiveDevelopment-test.js", which is very understandable and easy to pinpoint in the code. This is especially true when you look at a pair of lines like this:

```javascript
                expect("test/spec/LiveDevelopment-test.js").toBeInCache(matcher, "_specialsCache");
                expect("test/spec/LiveDevelopment-test.js").not.toBeInCache(matcher, "_noMatchCache");
```

If one of those fail, it would say something like "Expected test/spec/LiveDevelopment-test.js to be in the cache" and you wouldn't know which one actually failed.