This makes sure that all JS newlines are encoded in JSON.

It also moots a TODO about possibly escaping supplemental codepoints.
I served:

Content-Type: text/javascript;charset=UTF-8

var s = "%s";
document.write("<p>", s, "</p><ol>");
for (var i = 0; i < s.length; i++) {
  document.write("<li>", s.charCodeAt(i).toString(16), "</li>");
}
document.write("</l>");

where %s was replaced with bytes "\xf0\x9d\x84\x9e" to test
straight UTF-8 instead of encoding surrogates separately.

Recent Firefox, Chrome, and Safari all decoded it properly.
I have yet to try it on IE or older versions.

R=nigeltao
CC=golang-dev
https://golang.org/cl/5129042