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.)
I thought you didn't want to make `RuntimeString` reachable by default...