chore(test): migrate Dart tests to package:test

Instead of running with karma and the karma-dart shim, run dart
tests directly using the new package:test runner. This migrates
away from package:unittest.

Fixes a couple tests, mostly associated with depending on absolute
URLs or editing the test providers after an injector had already
been created.

Remove karma-dart and associated files. Change gupfiles to run tests
via `pub run test` instead.

modules/playground/test/benchpress/webdriver_async.server.spec.dart

@@ -1,6 +1,6 @@
 import 'dart:async';
 import 'dart:io' show Platform;
-import 'package:guinness/guinness.dart';
+import 'package:guinness2/guinness2.dart';
 import 'package:benchpress/benchpress.dart';
 import 'package:webdriver/webdriver.dart'
     show WebDriver, Capabilities, LogType, LogLevel, By;