Revert "[ffi]: Remove pointer elementAt method."

This reverts commit f706ff4ee2.

Reason for revert: b/321667799 - package:win32 uses this method - https://github.com/dart-windows/win32/blob/a78ff108fbf54927b0334c1bab8f15526c24cbf8/lib/src/com/iapplicationactivationmanager.dart#L46

Original change's description:
> [ffi]: Remove pointer elementAt method.
>
> Closes #54250
>
> TEST=test/ffi
>
> R=dacoharkes@google.com
> Change-Id: I0e88adfcfe3caef0ad3bb6814ad8f27dce5dc7f4
> CoreLibraryReviewExempt: FFI only
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/346380
> Reviewed-by: Daco Harkes <dacoharkes@google.com>
> Commit-Queue: Martin Kustermann <kustermann@google.com>
> Reviewed-by: Martin Kustermann <kustermann@google.com>
> Auto-Submit: Shikhar <shikharish05@gmail.com>

Change-Id: I1b7a48d14e9b85676a27f76a926e21cac9c76c85
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/347600
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>

samples/ffi/resource_management/arena_sample.dart

@@ -25,7 +25,7 @@ main() async {
   using((Arena arena) {
     final p = arena<Int64>(2);
     p[0] = 24;
-    MemMove((p + 1).cast<Void>(), p.cast<Void>(), sizeOf<Int64>());
+    MemMove(p.elementAt(1).cast<Void>(), p.cast<Void>(), sizeOf<Int64>());
     print(p[1]);
     Expect.equals(24, p[1]);
   });
@@ -35,7 +35,7 @@ main() async {
     using((Arena arena) {
       final p = arena<Int64>(2);
       p[0] = 25;
-      MemMove((p + 1).cast<Void>(), p.cast<Void>(), 8);
+      MemMove(p.elementAt(1).cast<Void>(), p.cast<Void>(), 8);
       print(p[1]);
       Expect.equals(25, p[1]);
       throw Exception("Some random exception");

samples/ffi/resource_management/arena_zoned_sample.dart

@@ -24,7 +24,7 @@ main() async {
   withZoneArena(() {
     final p = zoneArena<Int64>(2);
     p[0] = 24;
-    MemMove((p + 1).cast<Void>(), p.cast<Void>(), sizeOf<Int64>());
+    MemMove(p.elementAt(1).cast<Void>(), p.cast<Void>(), sizeOf<Int64>());
     print(p[1]);
     Expect.equals(24, p[1]);
   });
@@ -34,7 +34,7 @@ main() async {
     withZoneArena(() {
       final p = zoneArena<Int64>(2);
       p[0] = 25;
-      MemMove((p + 1).cast<Void>(), p.cast<Void>(), 8);
+      MemMove(p.elementAt(1).cast<Void>(), p.cast<Void>(), 8);
       print(p[1]);
       Expect.equals(25, p[1]);
       throw Exception("Some random exception");

samples/ffi/resource_management/unmanaged_sample.dart

@@ -25,7 +25,7 @@ main() {
   // For automatic management use a Arena.
   final p = calloc<Int64>(2);
   p[0] = 24;
-  memMove((p + 1).cast<Void>(), p.cast<Void>(), sizeOf<Int64>());
+  memMove(p.elementAt(1).cast<Void>(), p.cast<Void>(), sizeOf<Int64>());
   print(p[1]);
   Expect.equals(24, p[1]);
   calloc.free(p);

samples/ffi/sample_ffi_data.dart

@@ -71,7 +71,7 @@ main() {
     Pointer<Int32> p2 = p1.cast();
     print('${p2.runtimeType} value: ${p2.value}'); // -1
 
-    Pointer<Int32> p3 = p2 + 1;
+    Pointer<Int32> p3 = p2.elementAt(1);
     print('${p3.runtimeType} value: ${p3.value}'); // 2^31 - 1
 
     calloc.free(p1);
@@ -81,10 +81,10 @@ main() {
     // Data can be tightly packed in memory.
     Pointer<Int8> p = calloc(8);
     for (var i in [0, 1, 2, 3, 4, 5, 6, 7]) {
-      (p + i).value = i * 3;
+      p.elementAt(i).value = i * 3;
     }
     for (var i in [0, 1, 2, 3, 4, 5, 6, 7]) {
-      print('(p + $i) value: ${(p + i).value}');
+      print('p.elementAt($i) value: ${p.elementAt(i).value}');
     }
     calloc.free(p);
   }

samples/ffi/sample_ffi_functions.dart

@@ -197,7 +197,7 @@ main() {
     Pointer<Int64> p2 = calloc(2);
     p2.value = 42;
     p2[1] = 1000;
-    print((p2 + 1).address.toRadixString(16));
+    print(p2.elementAt(1).address.toRadixString(16));
     print(p2[1]);
     Pointer<Int64> result = assign1337Index1(p2);
     print(p2[1]);

samples/ffi/sample_ffi_functions_structs.dart

@@ -50,8 +50,8 @@ main() {
     NativeCoordinateOp f1 = p1.asFunction();
 
     Pointer<Coordinate> c1 = calloc<Coordinate>(3);
-    Pointer<Coordinate> c2 = c1 + 1;
-    Pointer<Coordinate> c3 = c1 + 2;
+    Pointer<Coordinate> c2 = c1.elementAt(1);
+    Pointer<Coordinate> c3 = c1.elementAt(2);
     c1.ref.x = 10.0;
     c1.ref.y = 10.0;
     c1.ref.next = c3;

samples/ffi/sample_ffi_structs.dart

@@ -40,8 +40,8 @@ main() {
   {
     // Allocates coordinates consecutively in c memory.
     Pointer<Coordinate> c1 = calloc<Coordinate>(3);
-    Pointer<Coordinate> c2 = c1 + 1;
-    Pointer<Coordinate> c3 = c1 + 2;
+    Pointer<Coordinate> c2 = c1.elementAt(1);
+    Pointer<Coordinate> c3 = c1.elementAt(2);
     c1.ref.x = 10.0;
     c1.ref.y = 10.0;
     c1.ref.next = c3;