Before, all test framework wrappers (internal for dart and js/ts,
angular2_test for dart and testing for js/ts) had similar logic to
keep track of current global test injector and test provider list.
This change wraps that logic into one class managed by the test
injector.
Closes#5819
angular2_testing is a user-facing dart test library built on top of
the package:test dart unittest framework and runner. For usage,
see modules_dart/angular2_testing/README.md.
Closes#3289
