Move a bunch of src/ files into subdirectories.

Move a bunch of src/ files into subdirectories.

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

Committed: https://github.com/dart-lang/unittest/commit/aede514bf3e112b05e942cdb264a0a7077e6821c

[kevmoo, 2015-02-24 04:46:49]

lgtm

[nweiz, 2015-02-24 05:03:57]

I forgot to fix the library tags; PTAL

[kevmoo, 2015-02-24 05:21:12]

lgtm

[nweiz, 2015-02-24 20:10:40]

Committed patchset #2 (id:20001) manually as
aede514bf3e112b05e942cdb264a0a7077e6821c (presubmit successful).