helpers nit: mark classes with only statics abstract final (#73)

Fix some relative imports

CHANGELOG.md

@@ -1,3 +1,5 @@
+## 0.2.1-wip
+
 ## 0.2.0-beta
 
 - Added `helpers.dart` top-level library.

lib/src/helpers/enums.dart

@@ -5,7 +5,7 @@
 /// Helper layer library that exposes enums commonly used from `dart:html`.
 library;
 
-import '../../web.dart';
+import '../dom/html.dart';
 
 abstract final class KeyLocation {
   // Note: this library uses legacy constant names for backwards compatibility.

lib/src/helpers/events/providers.dart

@@ -5,12 +5,12 @@
 import 'dart:js_interop';
 import 'dart:js_interop_unsafe';
 
-import '../../../helpers.dart' show Device;
 import '../../../web.dart';
+import '../enums.dart' show Device;
 import 'streams.dart';
 
 /// Statically accessible `EventStreamProvider`s for all event types.
-class EventStreamProviders {
+abstract final class EventStreamProviders {
   static const EventStreamProvider<ProgressEvent> abortEvent =
       EventStreamProvider<ProgressEvent>('abort');
 

pubspec.yaml

@@ -1,7 +1,7 @@
 name: web
 description: >-
   Lightweight DOM and JS bindings built around JS static interop.
-version: 0.2.0-beta
+version: 0.2.1-wip
 
 repository: https://github.com/dart-lang/web