[vm/ffi] Fix FFI Utf8 example.

Change-Id: Ic6c93c94f1187595b0dab2c912ebf4851ac85fe5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/108406
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Samir Jindel <sjindel@google.com>

samples/ffi/sqlite/lib/src/ffi/utf8.dart

@@ -9,7 +9,7 @@ import "arena.dart";
 
 /// Represents a String in C memory, managed by an [Arena].
 class Utf8 extends Struct<Utf8> {
-  @Int8()
+  @Uint8()
   int char;
 
   /// Allocates a [CString] in the current [Arena] and populates it with

samples/ffi/sqlite/test/sqlite_test.dart

@@ -7,6 +7,7 @@
 import "package:test/test.dart";
 
 import '../lib/sqlite.dart';
+import '../lib/src/ffi/utf8.dart';
 
 void main() {
   test("sqlite integration test", () {
@@ -161,4 +162,10 @@ void main() {
     r.close();
     d.close();
   });
+  test("Utf8 unit test", () {
+    final String test = 'Hasta Mañana';
+    final medium = Utf8.allocate(test);
+    expect(test, medium.load<Utf8>().toString());
+    medium.free();
+  });
 }

tests/ffi/object_gc_test.dart

@@ -12,7 +12,7 @@ import 'dart:ffi';
 
 import "package:expect/expect.dart";
 
-import 'cstring.dart';
+import 'utf8.dart';
 import 'dylib_utils.dart';
 
 DynamicLibrary ffiTestFunctions = dlopenPlatformSpecific("ffi_test_functions");

tests/ffi/structs_test.dart

@@ -12,6 +12,7 @@ import "package:expect/expect.dart";
 
 import 'coordinate_bare.dart' as bare;
 import 'coordinate.dart';
+import 'utf8.dart';
 
 void main() {
   testStructAllocate();
@@ -19,6 +20,7 @@ void main() {
   testStructWithNulls();
   testBareStruct();
   testTypeTest();
+  testUtf8();
 }
 
 /// allocates each coordinate separately in c memory
@@ -118,3 +120,10 @@ void testTypeTest() {
   Expect.isTrue(c is Struct<Coordinate>);
   c.addressOf.free();
 }
+
+void testUtf8() {
+  final String test = 'Hasta Mañana';
+  final Pointer<Utf8> medium = Utf8.toUtf8(test);
+  Expect.equals(test, Utf8.fromUtf8(medium));
+  medium.free();
+}

tests/ffi/utf8.dart

@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-library FfiTest;
+library Utf8;
 
 import 'dart:convert';
 import 'dart:ffi' as ffi;
@@ -10,7 +10,7 @@ import 'dart:ffi' show Pointer;
 
 /// Sample non-struct Pointer wrapper for dart:ffi library.
 class Utf8 extends ffi.Struct<Utf8> {
-  @ffi.Int8()
+  @ffi.Uint8()
   int char;
 
   static String fromUtf8(Pointer<Utf8> str) {
@@ -25,12 +25,13 @@ class Utf8 extends ffi.Struct<Utf8> {
   }
 
   static Pointer<Utf8> toUtf8(String s) {
-    Pointer<Utf8> result = Pointer<Utf8>.allocate(count: s.length + 1).cast();
     List<int> units = Utf8Encoder().convert(s);
-    for (int i = 0; i < s.length; i++) {
+    Pointer<Utf8> result =
+        Pointer<Utf8>.allocate(count: units.length + 1).cast();
+    for (int i = 0; i < units.length; i++) {
       result.elementAt(i).load<Utf8>().char = units[i];
     }
-    result.elementAt(s.length).load<Utf8>().char = 0;
+    result.elementAt(units.length).load<Utf8>().char = 0;
     return result;
   }
 }