Fix prefer_mixin TODOs (#136480)

Fixes https://github.com/flutter/flutter/issues/123172
Fixes https://github.com/dart-lang/sdk/issues/51813

In combination with https://github.com/flutter/flutter/pull/136418.

packages/flutter_tools/lib/src/base/error_handling_io.dart

@@ -156,8 +156,7 @@ class ErrorHandlingFileSystem extends ForwardingFileSystem {
 
 class ErrorHandlingFile
     extends ForwardingFileSystemEntity<File, io.File>
-    // TODO(goderbauer): Fix this ignore when https://github.com/google/file.dart/issues/209 is resolved.
-    with ForwardingFile { // ignore: prefer_mixin
+    with ForwardingFile {
   ErrorHandlingFile({
     required Platform platform,
     required this.fileSystem,
@@ -369,8 +368,7 @@ class ErrorHandlingFile
 
 class ErrorHandlingDirectory
     extends ForwardingFileSystemEntity<Directory, io.Directory>
-    // TODO(goderbauer): Fix this ignore when https://github.com/google/file.dart/issues/209 is resolved.
-    with ForwardingDirectory<Directory> { // ignore: prefer_mixin
+    with ForwardingDirectory<Directory> {
   ErrorHandlingDirectory({
     required Platform platform,
     required this.fileSystem,
@@ -506,8 +504,7 @@ class ErrorHandlingDirectory
 
 class ErrorHandlingLink
     extends ForwardingFileSystemEntity<Link, io.Link>
-    // TODO(goderbauer): Fix this ignore when https://github.com/google/file.dart/issues/209 is resolved.
-    with ForwardingLink { // ignore: prefer_mixin
+    with ForwardingLink {
   ErrorHandlingLink({
     required Platform platform,
     required this.fileSystem,

packages/flutter_tools/lib/src/base/multi_root_file_system.dart

@@ -209,8 +209,7 @@ abstract class MultiRootFileSystemEntity<T extends FileSystemEntity,
 }
 
 class MultiRootFile extends MultiRootFileSystemEntity<File, io.File>
-    // TODO(goderbauer): Fix this ignore when https://github.com/google/file.dart/issues/209 is resolved.
-    with ForwardingFile { // ignore: prefer_mixin
+    with ForwardingFile {
   MultiRootFile({
     required super.fileSystem,
     required super.delegate,
@@ -223,8 +222,7 @@ class MultiRootFile extends MultiRootFileSystemEntity<File, io.File>
 
 class MultiRootDirectory
     extends MultiRootFileSystemEntity<Directory, io.Directory>
-    // TODO(goderbauer): Fix this ignore when https://github.com/google/file.dart/issues/209 is resolved.
-    with ForwardingDirectory<Directory> { // ignore: prefer_mixin
+    with ForwardingDirectory<Directory> {
   MultiRootDirectory({
     required super.fileSystem,
     required super.delegate,
@@ -250,9 +248,9 @@ class MultiRootDirectory
     'MultiRootDirectory(fileSystem = $fileSystem, delegate = $delegate)';
 }
 
-class MultiRootLink extends MultiRootFileSystemEntity<Link, io.Link>
-    // TODO(goderbauer): Fix this ignore when https://github.com/google/file.dart/issues/209 is resolved.
-    with ForwardingLink { // ignore: prefer_mixin
+class MultiRootLink
+    extends MultiRootFileSystemEntity<Link, io.Link>
+    with ForwardingLink {
   MultiRootLink({
     required super.fileSystem,
     required super.delegate,