fix crash on web (dart:io)

lib/widgets/raw_editor.dart

@@ -1,6 +1,5 @@
 import 'dart:async';
 import 'dart:convert';
-import 'dart:io';
 
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/foundation.dart';
@@ -700,10 +699,10 @@ class RawEditorState extends EditorState
       handleDelete,
     );
 
-    if (Platform.isWindows ||
+    if (defaultTargetPlatform == TargetPlatform.windows ||
-        Platform.isMacOS ||
+        defaultTargetPlatform == TargetPlatform.macOS ||
-        Platform.isLinux ||
+        defaultTargetPlatform == TargetPlatform.linux ||
-        Platform.isFuchsia) {
+        defaultTargetPlatform == TargetPlatform.fuchsia) {
       _keyboardVisible = true;
     } else {
       _keyboardVisibilityController = KeyboardVisibilityController();