[sdk/vm] Make Base64._inverseAlphabet shared.

This is so that Base64 decoding is available for isolategroup-bound callbacks.

TEST=run_isolate_group_run_test
BUG=https://github.com/dart-lang/sdk/issues/61541
CoreLibraryReviewExempt: vm-only pragma
Change-Id: I30b8f0c36b0f980a1f5a13cec3cb07e214324ce0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/461180
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>

sdk/lib/convert/base64.dart

@@ -589,6 +589,7 @@ class _Base64Decoder {
   /// 62nd and 63rd alphabet characters), and considers `%` a padding
   /// character, which must then be followed by `3D`, the percent-escape
   /// for `=`.
+  @pragma('vm:shared')
   static final List<int> _inverseAlphabet = Int8List.fromList([
     __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
     __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //

tests/ffi/run_isolate_group_run_test.dart

@@ -13,6 +13,7 @@
 // VMOptions=--experimental-shared-data --profiler --profile_vm=false
 
 import 'package:dart_internal/isolate_group.dart' show IsolateGroup;
+import 'dart:convert';
 import 'dart:developer';
 import 'dart:isolate';
 
@@ -259,5 +260,12 @@ main(List<String> args) {
   });
   Expect.isTrue(resultIdentical);
 
+  Expect.listEquals(
+    "abcdefghijklmnopqrstuvwxyz".codeUnits,
+    IsolateGroup.runSync(() {
+      return Base64Decoder().convert("YWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXo=");
+    }),
+  );
+
   print("All tests completed :)");
 }