Assetfile.rb

require 'execjs'

class HandlebarsFilter < Filter
  class << self
    def contents
      @@contents ||= [File.read("headless-ember.js"), File.read("ember.js")].join("\n")
    end

    def context
      @@context ||= ExecJS.compile(contents)
    end
  end

  def generate_output(inputs, output)
    inputs.each do |input|
      name = File.basename(input.path, '.handlebars')
      compiled = self.class.context.call("precompileEmberHandlebars", input.read)
      output.write "\nEmber.TEMPLATES['#{name}'] = Ember.Handlebars.template(#{compiled});\n"
    end
  end
end

input  "templates"
output "output"

match "*.handlebars" do
  filter HandlebarsFilter
  concat "templates.js"
end

headless-ember.js

// DOM
var Element = {};
Element.firstChild = function () { return Element; };
Element.innerHTML = function () { return Element; };

var document = { createRange: false, createElement: function() { return Element; } };
var window = this;
this.document = document;

// Console
var console = window.console = {};
console.log = console.info = console.warn = console.error = function(){};

// jQuery
var jQuery = function() { return jQuery; };
jQuery.ready = function() { return jQuery; };
jQuery.inArray = function() { return jQuery; };
jQuery.jquery = "1.7.1";
var $ = jQuery;

// Ember
function precompileEmberHandlebars(string) {
  return Ember.Handlebars.precompile(string).toString();
}