Coverage for invalid RegExp source

ES5 has the following to say:

The characters / or backslash \ occurring in the pattern shall be escaped in S as necessary to ensure that the String value formed by concatenating the Strings "/", S, "/", and F can be parsed (in an appropriate lexical context) as a RegularExpressionLiteral that behaves identically to the constructed regular expression. For example, if P is "/", then S could be "/" or "\u002F", among other possibilities, but not "/", because /// followed by F would be parsed as a SingleLineComment rather than a RegularExpressionLiteral. If P is the empty String, this specification can be met by letting S be "(?:)"

The key is that var r = new RegExp(str); eval("/" + r.source + "/") should give you a regexp identical to r.

Implementations don't follow this too closely. IE doesn't handle empty string properly. FF/Chrome don't seem to handle "\n" properly. There are probably other interesting cases to be discovered.

Start by reviewing the ES5 rule quoted in the issue and the repository's existing RegExp tests. Add coverage for empty strings, newlines, slashes, backslashes, and other invalid or escaping cases, checking that eval("/" + r.source + "/") produces an equivalent regular expression.