Pattern.compile doesn't validate regex

In Java/Scala, `Pattern.compile` fails immediately on invalid regex.
In ScalaJS, it isn't until the pattern is used that it fails.

``` scala
// Doesn't throw in ScalaJS
val p = Pattern.compile("*")
```

To mimic JVM behaviour here (because I need to validate user-entered regex immediately), I just do this as a workaround:

``` scala
// Throws an exception
p.matcher("").matches()
```
