dillon1000/react

Avoid negative lookahead in regular expression

I'm trying to use React with otto
(https://github.com/robertkrimen/otto), which is implemented in Go and
does not support lookaheads or backreferences in regular expressions.
(Unfortunately that *does* mean that otto isn't fully compatible with
the ECMA5 spec; but since this is the only lookahead I am aware of in
the React codebase, this change makes React once again compatible with
otto, and any other JS implementation that may not be fully compliant on
regular expressions).

As far as I can tell, the previous code replaced a sequence of slash
characters with one more slash than before. The new code avoids the
negative lookahead by matching any sequence of slash characters and then
using the special `$&` placeholder to replace the matched sequence with
the original sequence, plus one more slash.