Fix #1252: Missing asInstanceOf's/unbox'es in scalajsenv.js.

Only one was really a danger, i.e., the result of `charCodeAt()` in `charSequenceCharAt()` with an out-of-bounds index. The others, being calls to methods of primitive data types, are hardly a problem, because the implementations cannot return wrong results. (Of course, they could be overwritten by an evil user, so it's better to check them.)
Why, just to not duplicate code? The optimizer won't be able to make this any faster, will it?