Escape backspace in regexp classes as "\b", not "\x08"

The "\x08" escaping is necessary only outside of classes (where "\b"
means "word boundary").

lib/compiler/js.js

@@ -42,6 +42,7 @@ var js = {
       .replace(/\^/g,   '\\^')    // caret
       .replace(/-/g,    '\\-')    // dash
       .replace(/\0/g,   '\\0')    // null
+      .replace(/\x08/g, '\\b')    // backspace
       .replace(/\t/g,   '\\t')    // horizontal tab
       .replace(/\n/g,   '\\n')    // line feed
       .replace(/\v/g,   '\\v')    // vertical tab