[io] Fix FileSystemEntity.type and Link.exists with IOOverrides

Fixes #63418

When IOOverrides are active, `fseGetType` and `fseGetTypeSync` used
`utf8.encode(path)` which does not null-terminate the path. Native
APIs require null-terminated paths. Through luck the sync path worked
but the async path failed with `notFound`.

Update them to use `FileSystemEntity._toUtf8Array(path)` which correctly
null-terminates the path.

Tested: added a regression test to tests/standalone/io/io_override_test.dart

TAG=agy
CONV=ab6af504-d536-4a8d-88be-bc487b60e24d
R=bkonyi@google.com

CoreLibraryReviewExempt: No API changes.
Change-Id: I24e31efdcbecc703800b96a144e41a095a445cff
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/505201
Reviewed-by: Ben Konyi <bkonyi@google.com>
Commit-Queue: Nate Bosch <nbosch@google.com>
Auto-Submit: Nate Bosch <nbosch@google.com>

sdk/lib/io/overrides.dart

@@ -259,7 +259,10 @@ abstract base class IOOverrides {
   /// When this override is installed, this function overrides the behavior of
   /// `FileSystemEntity.type`.
   Future<FileSystemEntityType> fseGetType(String path, bool followLinks) {
-    return FileSystemEntity._getTypeRequest(utf8.encode(path), followLinks);
+    return FileSystemEntity._getTypeRequest(
+      FileSystemEntity._toUtf8Array(path),
+      followLinks,
+    );
   }
 
   /// Returns the [FileSystemEntityType] for [path].
@@ -267,7 +270,10 @@ abstract base class IOOverrides {
   /// When this override is installed, this function overrides the behavior of
   /// `FileSystemEntity.typeSync`.
   FileSystemEntityType fseGetTypeSync(String path, bool followLinks) {
-    return FileSystemEntity._getTypeSyncHelper(utf8.encode(path), followLinks);
+    return FileSystemEntity._getTypeSyncHelper(
+      FileSystemEntity._toUtf8Array(path),
+      followLinks,
+    );
   }
 
   // _FileSystemWatcher

tests/standalone/io/io_override_test.dart

@@ -349,9 +349,22 @@ void emptyIOOverride() {
   );
 }
 
-main() async {
+Future<void> emptyIOOverrideAsync() async {
+  await IOOverrides.runWithIOOverrides(
+    () async => Expect.equals(
+      await FileSystemEntity.type('/'),
+      FileSystemEntityType.directory,
+    ),
+    EmptyOverride(),
+  );
+}
+
+void main() async {
+  asyncStart();
   await ioOverridesRunTest();
   globalIOOverridesTest();
   globalIOOverridesZoneTest();
   emptyIOOverride();
+  await emptyIOOverrideAsync();
+  asyncEnd();
 }