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

This reverts commit 57de39d.

Reason for revert: Breakage in flutter engine roll -- see: flutter/flutter#89406

Original change's description:
> [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>

TBR=vegorov@google.com,kustermann@google.com,aam@google.com,asiva@google.com

Change-Id: I4f20f3a998508b2465e5c16b64b363419da62775
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/212345
Auto-Submit: Brandon DeRosier <bdero@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

Author: bdero

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=--no-enable-isolate-groups
+// VMOptions=
 // 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=--no-enable-isolate-groups
+// VMOptions=
 // VMOptions=--enable-isolate-groups --disable-heap-verification
 
 import 'dart:async';

runtime/vm/flag_list.h

@@ -196,7 +196,8 @@ 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, true, "Enable isolate group support.")        \
+  P(enable_isolate_groups, bool, false,                                        \
+    "Enable isolate group support in AOT.")                                    \
   P(show_invisible_frames, bool, false,                                        \
     "Show invisible frames in stack traces.")                                  \
   D(trace_cha, bool, false, "Trace CHA operations")                            \