[vm] Fix default log filtering to include the vm-service.

This was probably broken by some refactoring related to isolate groups.

TEST=--disassemble-stubs
Change-Id: If46259836d2276d50fefc5598f8370bf935ec95e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/244481
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Ben Konyi <bkonyi@google.com>
diff --git a/runtime/vm/log.cc b/runtime/vm/log.cc
index 809df77..70669b3 100644
--- a/runtime/vm/log.cc
+++ b/runtime/vm/log.cc
@@ -161,8 +161,11 @@
 
 bool Log::ShouldLogForIsolateGroup(const IsolateGroup* isolate_group) {
   if (FLAG_isolate_log_filter == nullptr) {
+    // By default, do not log for the service or kernel isolates.
+    if (isolate_group == Dart::vm_isolate_group()) {
+      return true;
+    }
     if (IsolateGroup::IsSystemIsolateGroup(isolate_group)) {
-      // By default, do not log for the service or kernel isolates.
       return false;
     }
     return true;