Add simple top level Dart console app for running unit tests.

During the json parser implementation, I ended up creating a simple Dart console app to run unit tests. It is not meant as a replacement for Spark test, but simply as a convenience for devs writing tests for simple utility/low level code which does not require the whole Spark app to run. For now, we use the import + file path to include the test files, so this is not ideal, but it works.

Pros:
* These tests don't need to run as a Chrome App, so they run fast.
* There tests can be debugged "natively" inside the Dart Editor, making debugging easier and more efficient.

Cons:
* You have to include the test file manually (in addition to including it in "all_test.dart" to run them in Spark).

@devoncarew