I think `isEmpty()` would be better off not optionally trimming whitespace, since that can easily be done by the caller.

The intent of a test like:

```js
Y.Test.StringAssert.isEmpty(Y.Lang.trim(foo));
```

...is more clear at a glance than:

```js
Y.Test.StringAssert.isEmpty(foo, true);
```

...which is a [boolean trap](http://ariya.ofilabs.com/2011/08/hall-of-api-shame-boolean-trap.html).

We don't need to worry about the behavior of `Y.Lang.trim()` on non-string values, since it will return them unmodified.

I also can't think of an existing assertion method whose behavior changes based on an optional parameter, and I think it's a good idea to maintain that consistency.