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.

tools/build/create_dart_test_main.js

@@ -0,0 +1,21 @@
+var path = require('path');
+var fs = require('fs');
+
+module.exports = function(dir, files) {
+  var filename = 'main_test.dart';
+  var imports = [
+    '@TestOn("browser")',
+    'import "package:guinness2/guinness2.dart";'];
+  var executes = [];
+
+  files.forEach(function(match) {
+    var varName = match.replace(/[\/.]/g, '_');
+    imports.push('import "' + match + '" as ' + varName +';');
+    executes.push('  ' + varName + '.main();');
+  });
+
+  var output = imports.join('\n') + '\n\nmain() {\n' + executes.join('\n') + '\n}';
+
+  fs.writeFileSync(path.join(dir, filename), output);
+  return filename;
+};