Switched to injecting code into iframes asynchronously via script tags rather than synchronously via window.eval(), b/c the latter didn't always result in proper evaluation (e.g., when "foo = 1" was eval'd, 'foo' was attached to the global object of the caller rather than the window object). Also added fake Observers and Preferences JS modules.

let EXPORTED_SYMBOLS = ["Preferences"];

function Preferences() {
  this.get = function() {
  };
}