Better error (with watching!) for missing source files.
Export JSON from json package.

Make sure to use the built-in JSON if it exists.

Fixes #1204.
Use json package in sockjs client.

Because sockjs-client doesn't think it's loading in a closure, it ALWAYS ends up
redefining JSON to non-native code, and sockjs-client does not want to change
this:
  https://github.com/sockjs/sockjs-client/issues/123

Fixes #1162.
Don't print a doubled slash if it's included in ROOT_URL.

This should not affect how Meteor.absoluteUrl actually works since it adds a
slash to the end of the root URL if it doesn't have one already.
Avoid using Meteor.absoluteUrl for client-side tests.

This way tests can pass both when ran against localhost or against a
10.0.2.2-style hostname for VMs.
minor improvements to a livedata test
clear all sockjs handlers