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

Various VM embedders have already explicitly opted into it
AOT mode for a long time.

Now we turn it on by default, also because we'd like to collect
feedback from the field on this - in good time before the next
stable branch will be released.

All of our isolate related tests have already

    // VMOptions=--enable-isolate-groups ...
    // VMOptions=--no-enable-isolate-groups ...

So they will continue to exercise the tests in both modes.

Issue #36097

TEST=Existing test suite.

Change-Id: I1d031e8a5f2173b48eb32c53b08daccda75dc51d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/208649
Reviewed-by: Slava Egorov <vegorov@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

Author: mkustermann

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")                            \