Create a set of Dart dockerfiles

These dockerfles are designed to follow the same patters as the ones for
nodejs, Python and Go at

  https://github.com/GoogleCloudPlatform/nodejs-docker
  https://github.com/GoogleCloudPlatform/python-docker
  https://github.com/GoogleCloudPlatform/golang-docker

R=kevmoo@google.com, kustermann@google.com, proppy@google.com
BUG=

Review URL: https://codereview.chromium.org//367903008