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.)