Move IterableSet to test_core (#1489)

This utility was not used in `test_api`. Move to `lib/src/runner/util`
since it is only used from one place under `lib/src/runner`.

Run dartfmt with the latest SDK.
diff --git a/pkgs/test_core/lib/src/executable.dart b/pkgs/test_core/lib/src/executable.dart
index 12f13ec..c6814ec 100644
--- a/pkgs/test_core/lib/src/executable.dart
+++ b/pkgs/test_core/lib/src/executable.dart
@@ -62,9 +62,9 @@
   final _signals = Platform.isWindows
       ? ProcessSignal.sigint.watch()
       : Platform.isFuchsia // Signals don't exist on Fuchsia.
-          ? Stream.empty()
-          : StreamGroup.merge(
-              [ProcessSignal.sigterm.watch(), ProcessSignal.sigint.watch()]);
+      ? Stream.empty()
+      : StreamGroup.merge(
+          [ProcessSignal.sigterm.watch(), ProcessSignal.sigint.watch()]);
 
   Configuration configuration;
   try {
diff --git a/pkgs/test_core/lib/src/runner/configuration/reporters.dart b/pkgs/test_core/lib/src/runner/configuration/reporters.dart
index 331a113..3d4f2ad 100644
--- a/pkgs/test_core/lib/src/runner/configuration/reporters.dart
+++ b/pkgs/test_core/lib/src/runner/configuration/reporters.dart
@@ -52,8 +52,8 @@
 final defaultReporter = inTestTests
     ? 'expanded'
     : canUseSpecialChars
-        ? 'compact'
-        : 'expanded';
+    ? 'compact'
+    : 'expanded';
 
 /// **Do not call this function without express permission from the test package
 /// authors**.
diff --git a/pkgs/test_core/lib/src/runner/engine.dart b/pkgs/test_core/lib/src/runner/engine.dart
index 27af38d..2973d76 100644
--- a/pkgs/test_core/lib/src/runner/engine.dart
+++ b/pkgs/test_core/lib/src/runner/engine.dart
@@ -16,13 +16,13 @@
 import 'package:test_api/src/backend/message.dart'; // ignore: implementation_imports
 import 'package:test_api/src/backend/state.dart'; // ignore: implementation_imports
 import 'package:test_api/src/backend/test.dart'; // ignore: implementation_imports
-import 'package:test_api/src/util/iterable_set.dart'; // ignore: implementation_imports
 
 import 'coverage_stub.dart' if (dart.library.io) 'coverage.dart';
 import 'live_suite.dart';
 import 'live_suite_controller.dart';
 import 'load_suite.dart';
 import 'runner_suite.dart';
+import 'util/iterable_set.dart';
 
 /// An [Engine] manages a run that encompasses multiple test suites.
 ///
diff --git a/pkgs/test_api/lib/src/util/iterable_set.dart b/pkgs/test_core/lib/src/runner/util/iterable_set.dart
similarity index 100%
rename from pkgs/test_api/lib/src/util/iterable_set.dart
rename to pkgs/test_core/lib/src/runner/util/iterable_set.dart