[sdk/vm] Use FinalThreadLocal to store _Random singleton object.

This allows use of Random in isolategroup-bound callbacks.

BUG=https://github.com/dart-lang/sdk/issues/61541
TEST=run_isolate_group_run_test

Change-Id: Id847e900be05f2866386c025b1796b3f34c1e8df
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/461940
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>

sdk/lib/_internal/vm/lib/math_patch.dart

@@ -10,6 +10,7 @@
 import "dart:_internal" show patch;
 
 import "dart:typed_data" show Uint32List;
+import "dart:_vm" show FinalThreadLocal;
 
 /// There are no parts of this patch library.
 
@@ -245,7 +246,10 @@ class _Random implements Random {
   static const _POW2_27_D = 1.0 * (1 << 27);
 
   // Use a singleton Random object to get a new seed if no seed was passed.
-  static final _prng = _Random._withState(_initialSeed());
+  @pragma('vm:shared')
+  static final _prng = FinalThreadLocal<_Random>(
+    () => _Random._withState(_initialSeed()),
+  );
 
   // Thomas Wang 64-bit mix.
   // http://www.concentric.net/~Ttwang/tech/inthash.htm
@@ -270,8 +274,8 @@ class _Random implements Random {
 
   static int _nextSeed() {
     // Trigger the PRNG once to change the internal state.
-    _prng._nextState();
-    return _prng._state & 0xFFFFFFFF;
+    _prng.value._nextState();
+    return _prng.value._state & 0xFFFFFFFF;
   }
 }
 

tests/ffi/run_isolate_group_run_test.dart

@@ -16,6 +16,7 @@ import 'package:dart_internal/isolate_group.dart' show IsolateGroup;
 import 'dart:convert';
 import 'dart:developer';
 import 'dart:isolate';
+import 'dart:math';
 
 import "package:expect/expect.dart";
 
@@ -267,5 +268,9 @@ main(List<String> args) {
     }),
   );
 
+  IsolateGroup.runSync(() {
+    Random().nextInt(10);
+  });
+
   print("All tests completed :)");
 }