fix[web]: unregister previously registered clipboard event listeners (#528)

Fixes #507

This happens because the dom listeners are not unregisterd at hot
reload. The best way for this would be if flutter exposed `registerHotRestartListener` 
so we could have proper cleanup, but that's not the case. As a workaround we can
 store previous listeners in `window` object and unregister them during initialization.

Author: knopp

super_native_extensions/lib/src/web/clipboard_events.dart

@@ -1,5 +1,5 @@
 import 'dart:js_interop';
-
+import 'dart:js_interop_unsafe';
 import 'package:web/web.dart' as web;
 
 import '../clipboard_events.dart';
@@ -73,10 +73,29 @@ class _WriteEvent extends ClipboardWriteEvent {
 }
 
 class ClipboardEventsImpl extends ClipboardEvents {
+  static const listenersProperty =
+      "super_native_extensions_clipboard_events_listeners";
+
   ClipboardEventsImpl() {
-    web.window.addEventListener('paste', _onPaste.toJS);
-    web.window.addEventListener('copy', _onCopy.toJS);
-    web.window.addEventListener('cut', _onCut.toJS);
+    // Have no access to `registerHotRestartListener` so this needs to be done manually on startup.
+    {
+      final listeners = web.window.getProperty(listenersProperty.toJS)
+          as JSArray<JSListener>?;
+      if (listeners != null) {
+        for (final listener in listeners.toDart) {
+          web.window.removeEventListener(listener.type, listener.callback);
+        }
+      }
+    }
+    final listeners = [
+      JSListener(type: 'paste', callback: _onPaste.toJS),
+      JSListener(type: 'copy', callback: _onCopy.toJS),
+      JSListener(type: 'cut', callback: _onCut.toJS),
+    ];
+    for (final listener in listeners) {
+      web.window.addEventListener(listener.type, listener.callback);
+    }
+    web.window.setProperty(listenersProperty.toJS, listeners.toJS);
   }
 
   @override