This creates the following four subdirectories:

* `frontend/`, which is for user-visible libraries exported by
  `lib/unittest.dart`.

* `backend/`, which is for libraries that define the internal
  structure of test suites and how they're run in a platform-
  independent manner.

* `runner/`, which is for libraries that are specifically useful for
  the test runner. This includes code that deals with multiple test
  suites at once as well as platform-specific test running code.

* `util/`, which contains various utility libraries that aren't
  specific to unittest's use-case. Individual utility functions can
  still go in `lib/util.dart`.

* `deprecated/`, which contains deprecated libraries.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//955543002