[core/shared] Mark _CopyingBytesBuilder._emptyList shared

This enables use of BytesBuilder in isolategroup-bound callbacks.

TEST=run_isolate_group_run_test
CoreLibraryReviewExempt: this adds vm-specific pragma
Bug: https://github.com/dart-lang/sdk/issues/61030
Change-Id: Ic73887af099e6a7636e939d378dbe08a2f0c8f98
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/476601
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>

sdk/lib/internal/bytes_builder.dart

@@ -74,6 +74,7 @@ class _CopyingBytesBuilder implements BytesBuilder {
   /// Reusable empty [Uint8List].
   ///
   /// Safe for reuse because a fixed-length empty list is immutable.
+  @pragma('vm:shared')
   static final _emptyList = Uint8List(0);
 
   /// Current count of bytes written to buffer.

tests/ffi/run_isolate_group_run_test.dart

@@ -15,6 +15,7 @@
 import 'package:dart_internal/isolate_group.dart' show IsolateGroup;
 import 'dart:convert';
 import 'dart:developer';
+import 'dart:io';
 import 'dart:isolate';
 import 'dart:math';
 
@@ -68,6 +69,8 @@ main(List<String> args) {
   testEncoding();
   testRecursiveToString();
 
+  testBytesBuilder();
+
   print("All tests completed :)");
 }
 
@@ -523,3 +526,12 @@ void testRecursiveToString() {
     }),
   );
 }
+
+///
+void testBytesBuilder() {
+  BytesBuilder builder = new BytesBuilder();
+  builder.add([1, 2, 3]);
+  Expect.listEquals([1, 2, 3], builder.toBytes());
+  builder.clear();
+  Expect.isTrue(builder.isEmpty);
+}