title: how can we use environment variable in angular version >=2
body: <p>I am now reading the angular4 doc, and noticed, there is one folder, environments, and under which, there are multiple environment files, such as environment.ts, environment.prod.ts, if we build using env=prod, then prod environments variable will be used.
  Now I have one question, how can I get environment variable from the code? I googled the doc quite a lot and there is no mention about this.
  Can anyone help to tell me how to obtain environment variable values?
  Thanks</p>