Add "base" library. Add checked mode tester.
Made checked mode check an assert.
Renamed base library to testing.
Moved async and time test code under testing.
Throw StateError explicitly instead of using assert. Document thrown exception.
Moved lib/src/testing to lib/testing/src. Moved top level testing files from lib/ to lib/testing.
Added top level runtime file.
Fixed checked_mode comment, added testing/all_tests comment.
Merge pull request #30 from davidmorgan/add-base-checked-mode

Add "base" library. Add checked mode tester.