Reland "[vm/concurrency] Enable isolate groups by-default in all modes"

This reverts commit d0a3daf that
reverted the initial land.

The fix for the problem that required the revert landed in
https://dart.googlesource.com/sdk/+/5ccf97a5a950ef70f41d84f186dad550d4acda01

Issue #36097

TEST=ci

Change-Id: Ie92074059319e046809169d9e16d459ce1c2848f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/212481
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

runtime/tests/vm/dart/isolates/dart_api_create_lightweight_isolate_test.dart

@@ -3,7 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 // SharedObjects=ffi_test_functions
-// VMOptions=
+// VMOptions=--no-enable-isolate-groups
 // VMOptions=--enable-isolate-groups --disable-heap-verification
 
 import 'dart:async';

runtime/tests/vm/dart_2/isolates/dart_api_create_lightweight_isolate_test.dart

@@ -5,7 +5,7 @@
 // @dart = 2.9
 
 // SharedObjects=ffi_test_functions
-// VMOptions=
+// VMOptions=--no-enable-isolate-groups
 // VMOptions=--enable-isolate-groups --disable-heap-verification
 
 import 'dart:async';

runtime/vm/flag_list.h

@@ -196,8 +196,7 @@ constexpr bool FLAG_support_il_printer = false;
   P(retain_code_objects, bool, true,                                           \
     "Serialize all code objects even if not otherwise "                        \
     "needed in the precompiled runtime.")                                      \
-  P(enable_isolate_groups, bool, false,                                        \
-    "Enable isolate group support in AOT.")                                    \
+  P(enable_isolate_groups, bool, true, "Enable isolate group support.")        \
   P(show_invisible_frames, bool, false,                                        \
     "Show invisible frames in stack traces.")                                  \
   D(trace_cha, bool, false, "Trace CHA operations")                            \