Register Dart with the asset pipeline.

railtie.rb

module Dart
  module Rails
    class Railtie < ::Rails::Engine

      initializer :setup_dart_rails do |app|
        app.assets.register_engine '.dart', Dart::Rails::Template
      end

    end
  end
end

template.rb

require 'fileutils'
require 'open3'

module Dart
  module Rails
    class Template < Tilt::Template
      include TiltCompat

      def render(scope, locals, &block)
        @data = dart(@file)
      end

    protected

      def compile_target
        @compile_target ||= File.open(File.join(cache_dir, 'output.js'), 'w')
      end

      def cache_dir
        @cache_dir ||= ::Rails.root.join('tmp/cache/dart-rails').tap do |dir|
          FileUtils.rm_r(dir) if Dir.exist?(dir)
          Dir.mkdir(dir)
        end
      end

      def dart(file, options = {})
        bin     = ENV['DART2JS_BIN'] || '/usr/bin/env dart2js'
        command = "#{bin} --out=#{compile_target.path} #{file}"

        stdin, stdout, stderr = Open3.popen3(command)

        if error = stdout.gets
          raise error
        end

        stdin.close
        stdout.close
        stderr.close

        `cat #{compile_target.path}`
      end

      def logger
        ::Rails.logger
      end

    end
  end
end

tilt_compat.rb

module Dart
  module Rails
    module TiltCompat

      class << self

        def engine_initialized?
          true
        end

      end

      def prepare
        true
      end

    end
  end
end