[doc/ffi] Fix CString implementation in dart:ffi sample

Change-Id: I3dcb9fefe3d13f21da1c923dd69cc2a994ccf51e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/103128
Auto-Submit: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>

samples/ffi/sqlite/docs/sqlite-tutorial.md

@@ -71,19 +71,19 @@ class StatementPointer extends Pointer<Void> {}
 Strings in C are pointers to character arrays.
 
 ```dart
-class CString extends Pointer<Int8> {}
+class CString extends Pointer<Uint8> {}
 ```
 
 Pointers to C integers, floats, an doubles can be read from and written through to `dart:ffi`.
 However, before we can write to C memory from dart, we need to `allocate` some memory.
 
 ```dart
-Pointer<Int8> p = allocate(); // Infers type argument allocate<Int8>(), and allocates 1 byte.
-p.store(123);                 // Stores a Dart int into this C int8.
-int v = p.load();             // Infers type argument p.load<int>(), and loads a value from C memory.
+Pointer<Uint8> p = allocate(); // Infers type argument allocate<Uint8>(), and allocates 1 byte.
+p.store(123);                  // Stores a Dart int into this C int8.
+int v = p.load();              // Infers type argument p.load<int>(), and loads a value from C memory.
 ```
 
-Note that you can only load a Dart `int` from a C `Int8`.
+Note that you can only load a Dart `int` from a C `Uint8`.
 Trying to load a Dart `double` will result in a runtime exception.
 
 We've almost modeled C Strings.

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

@@ -8,7 +8,7 @@ import "dart:ffi";
 import "arena.dart";
 
 /// Represents a String in C memory, managed by an [Arena].
-class CString extends Pointer<Int8> {
+class CString extends Pointer<Uint8> {
   /// Allocates a [CString] in the current [Arena] and populates it with
   /// [dartStr].
   factory CString(String dartStr) => CString.inArena(Arena.current(), dartStr);
@@ -23,7 +23,7 @@ class CString extends Pointer<Int8> {
   /// memory manually!
   factory CString.allocate(String dartStr) {
     List<int> units = Utf8Encoder().convert(dartStr);
-    Pointer<Int8> str = allocate(count: units.length + 1);
+    Pointer<Uint8> str = allocate(count: units.length + 1);
     for (int i = 0; i < units.length; ++i) {
       str.elementAt(i).store(units[i]);
     }